Notice: This material may be protected by
copyright law (Title 17 U.S. Code)
Pursuant to the federal copyright act (Title 17 of the United
States Code), it is presumptively unlawful to reproduce,
distribute, or publicly display any copyrighted work (or any
substantial portion thereof), without the permission of the
copyright owner. The statute, however, recognizes a fair use
defense that has the effect of excusing an act of copyright
infringement. It is the intention of the library to act within the
parameters of the fair use defense in allowing limited posting of
copyrighted materials in Electronic Course Reserve areas such
as this one. It is the intention of the library, moreover, that such
materials be made available solely for the purposes of private
study, scholarship, and research, and that any further
reproduction of such materials by students, by printing or
downloading, be limited to such purposes. Any further
reproduction of copyrighted materials made from this computer
system may be in violation of copyright laws and is prohibited.
Chapter Three
3
P RO LO G U E:
The Start of Life: G e netics
and Prenatal Deve lo p me nt
TH E F UTU R E I S N O W
It came out of the blue: Jana and To m Mon aco's seem i ngl y healthy
3-year-old son Steph n developed a l i fe-threaten i n g stomach virus
that led to severe brain damage . His d i agnosis: a ra re b u t t reatable
di ease called isovaleric acidemia ( I VA ) , marked by the body'
inabili ty to metabolize an am ino acid fo u n d i n dietary protein.
Jana and Tom were unkn owing ca rrier of the d i sease . . . . The
Monacos had no warn i n g whatsoever.
N ot so when Jana got p regnant again. Her da ughter, Caro l i ne,
was tested by amnjocen tesis while still in the womb. Knowing ar­
oline had the mutation, doctors were able to ad m i n i ter med ica­
tion the day he was born-and the Monacos were prepared to
monitor her diet i mmediately to keep her healthy. Today tephen,
9, is unable to walk, talk or feed him elf. Caro l i ne, meanwh i le, i s an
active, heal thy 4-year-old. Genetic te ting, say Jana, "gives Caro­
line the future that Stephen didn't gel to have." ( Kalb, 2006, p. 52)
A h i d d e n ge n et i c d i s o rder robbed Jana a n d Tom M o n a o'
fi rst ch i l d o t a n o r m a l , healthy l i fe. Their e cond c h i l d w a s
pared Lhe
a m e fate b y
advances i n ge n et i c test i n g,
w h i c h gave the M o n acos a
c h a n ce l o i n lerve n e before t h e
...--d a m a ge v. a s d o n e. T h e y were
a b l t: to \ !O p Caro l i n e' i n h er i t ed
d i -,o rd e r fro m d o i n g 1 h e sa m e
d a m age b , co n t ro l l i n g aspec t s
o f h e r e n , i rn n m en t .
• \,\f1iat is o u r basic ge11et ic e11do w111e11t, a ml h o w rt1 11
lz 11mrrn de 1 1elopme111 go ojf t rack?
• How clo t/1e C!lll·iro11 111cnl mid ge11et ics work /o�t•lhcr to
del er111 i rie h u m a n cl1t1 rnctl!rist ics?
CHAP�E R OUTLI N E
Multiple Births: Two-or More-For
the Genelic Pnce of One
Inherited and Genetic Disorders:
When Development Goes Awry
Prologue: Th e F11 1 11 re Is No w
The Basics of Genetics. The M1x1ng
and Matching of Traits
Genetic Counseling: Predicting the
Future From t1,e Genes of the
Present
L o o k i ng A h ea d
Etuliest Develop ment
Genes and Chromosomes·
n,e: Gode o Lil&
44
I n t h i s ch,, p l cr, wt· ' I I t·xa m i n t• 1\' 1 1 ,1 1 d t• ,·d o p m c n l a l
resea rc h e r '> a n d o t h c r ,t·i e n l i !-> h h a vc k.1 r 1 1 nl a hu u t 1,·.1 �· , 1 h a t
h e red i t y a n d t h e t· 1w i ro n m e 1 1 1 w o r k i n 1 ,1 1 1 <l c 1 1 1 t o c r ...·,1 1 1.: .1 1 1 d
� h a pc h u m a n b e i n g, . \\'c beg i n \\' i t h l h c ba !, i c ., o f' h e rcd i t ) , t h e
gcn c t ic t r a n ., m i !, '> i o n o f ch a rac t c r i ... 1 i c ., fro m h i o l o g i t a l p a rc n h
I < > t h e i r ch i l d re n , by ex a m i n i n g h o \\' w c rccc i \ c o u r !..:''- J l l' l it
e n d o w m e n t . We co n , i d c r beh a v i o ra l gcnt·t i ·,, an a rl',1 of '>t u J �·
t h a t ,pcci a l i7cs i n t h e co n ,cq u e n ccs o f h l' rcd i t y 0 11 lwh ,I \ i o r. \\'c
a l so d i'>C LI � , w h a t h a p pc n !, w h e n gcn c t i c fa t.:1 o r-, ca u ., ,._. d c \·d o p ­
m e n t t o go ,n,· r�·, a n d h o w '>llch p ro b l c m '> a rc d ca l 1 1, i t h t h n , u g h
ge n e t i c coL1 11 ,c: l i 11 g .i n d gt· n c t h era p y.
B u t gc n c '> a rc o n l y 0 1 1 c pa rt o f t h e , t o r y o f p re 1 1 a l ,1 I ti c , d n p ­
m c n t . We a l ,o c o n s i d e r t h e wa y:- i n w h i d 1 a c h i l d \ gc n c l i l' h e r­
i t a ge i 11 1 crac 1 w i t h h e r e n v i ro n m e n t : I n o t h e r wo rd , , h o " o n c\
fo m i l y, '>oc i occo n o m ic � l n l u �, a n d J i fl' t' \'Cl l h ca n a ffec1 .1 ,·a r i l' l y
o f c h a ra c t e r i s t i c!,, i n c l u d i n g p hy -. i c a l t ra i h, i 11 1 c l l i gt· tKt' , ,1 11 d
even pcr!,ona I i t y.
h n a l l y, we foc.:t1 , o n t h e ver y fi rq , t age o f d • n· l o p 111 c n l ,
t rac i n g p rc n a l a l g ro \\• t h a n d drn n gc. \Ve n· , i c \\· <,lJflll' o f t h e
a l t e rn a t i ve:,, ava i l a b l e l o o u p l e:-, w h 1 1 fi n d i t d i ffi c u l 1 t o co 1 1 1: ...· i v c .
\\I� a l �o t a l k ,1 b o u 1 t h e � t a �c� o r 1 h c p n• m1 1 a l p e r i o d ,1 11d l w w t h e
p ren a t a l env i ro n m e n t ffl'r<, bot h 1 h r ·a l '> t o-a n d t h e p ro m i se
of-fu t u re g ro w 1 h .
A ft e r read i n g t h i � c h a p t e r, ycrn w i l l b c a h l c t o a 1 h \1·c r t l H::-.c
q uc:.t i o n s:
Transmission of Genetic Information
The Human Genome and Behavioral
Geneltcs· Cracking the Genetic
Code
Review
The I n te rc;tctio n
of H erec;(. i t� ctnd
E nvironmeVL t
FROM R E SE A R C H T O PRACTICE:
Are "Designer Babies" in O u r Future?
The Role of the Env1ronm nt
1n Deter1111111ng the Expression
of Genes. From Genotypes lo
Phenotypes
Studying Development: How Mucl1
Is Nature? How Much Is Nurture?
Pl,ysical Trails: Family
Resemblances
Intelligence: More Research. More
Controversy
Genetic and Environmental
Influences on Personality· Born to
B Outgoing?
DEVELOPMENTAL D IVERSITY:
Cultural Differences in Physical Arousal:
Might a Cullure's Philosophical Outlook Be
Determined by Genet ics?
Psychological Disorders: The Role of
Genetics and Environment
Can Genes Influence ll1e
Environment?
Review
Pre vtettet l G row th
etncl C h et n !!.J e
Fertilization: The Moment of
Conception
The Stages of the Prenatal Penod:
Tlie Onset of Development
Pregnancy Problems
Tl1e Prenatal Environmem. Threats
to Development
B E CO M I N G AN I N FO R M ED CONSU M E R
OF D EVELOPM ENT:
Optimizing the Prenatal Environment
Review
T H E CASE OF. . . The Genetic Finger
of Fate
L o o k i 11g BMk
Ep ilog ut!
Key Tc: r111 s n 1 1rl
0 1 1 n• p ts
45
Gametes The sex cells from
the mother and father that form a
new cell at conception
Zygote The new cell formed
by the process of ferti lization
Genes The basic u n it of
genetic information
DNA (deoxyribonucleic acid}
molecules The substance that
genes are composed of that
determines the nature of every cell
in the body and how it will function
C h ro m o s o m e s Rod-shaped
portions of ONA that are
organ ized in 23 pairs
M o n ozygoti c tw i ns Twins
who a re genetically identical
We humans be gin the course of our lives simply.
Like individuals from tens of thousands of other species,
we start as a single cell, a tiny speck probably weigh i n g no
more than 1 /2 0 -milJionth of an ounce. But from this h u m ble
beginning, in relatively few months, a l iving, bre ath i ng,
un ique infant is born. This fi rst cell is created when a mal e
reproductive cell , a sperm, pushes thro u gh the membrane of
the ovum, the female reproductive cell. .T hese g� as the
male and fem le._rep.w ductiv-e c:ells are als
,vn each con ­
tain huge amounts o enetic i n form ation . About an �r o r
s o aft e r the sperm enters the ov um, the two gametes suddenly
fuse becomi g_one-Gellra-zr.gore: The resulti ng combinat io n
of their genetic instructions-ove r 2 billi o n chemical ly coded
messages-is sufficient to begin crea ti n g a whole person .
pcc i fic ge nes in p rec ise l oca t i o n s o n the ·h a i n o f chro­
mo o m es determ i n e the n a t u re and fu n c t i o n 1 f c,·cr r el l in
the body. Fo r in l a n ce, ge nes detcrm i n l' w h i c h ·l'I I \\· i l l u l t i ­
m a t e l } b c me pa r t o f t h e h ea r t a n d wh ich \\" i l l become part
of t h e mu cles of th leg. G e n es a lso l':-t a b l i :, h how d i fferent
pa r t of t h e body w i l l fu n c t i o n : h m ra i I I }' t h l' heart ,, ill
beat, o r how m uch s t rength a m u ·ell' , i l l h a ve.
If each pare n t prov ides j ust 23 c h rn rn o ·n m cs, ,, here
doe t h e poten t i a l for t h e vast d i ve rs i t y or h u ma n being
come fro m ? The a n swer resi des p r i m a r i l y i n t h e n a t u re of
the p rocesses t h a t u n derl ie t h e ce l l d i v i s i o n or t h e gametes.
v\fhen t h e sperm a n d ova a re formed in t h e ad u l t h u man
body in a pro cess ca l l ed meiosis, each ga m e t e rece ives one of
the two chromoso mes that m a ke u 1 each o r t l1l· 23 pa irs.
Beca u e for each o f t h e 23 pa i rs i t i s la rge!} a m a t ter of
cha nce which m e m ber o f the pa i r is co n t r i h u t l·d, t he re are
2- 3 , or about 8 m i l l ion com b i na t ions poss i ble. Fu r t h ermore,
other p roccsse , such as random t ra n s fo r m a t i o ns or pa r t icu­
lar genes, acid to the va r i abi l i t y o f the gen e t i c brew. The u l t i ­
m a t e ou tcome: tens o f t rillio11s of poss i b l e genetic
com b i nations.
With so many poss i b l e ge n e t i c m i x t u res p rovi ded by
hered i t y, there is no l i kel i h ood t h a t someday yo u' l l b u m p
i n t o a ge n e t i c d u p l i ca te of you rse l f-w i t h o n e excep t io n : an
identical t w i n .
Genes and Chromoso m es:
The Code of Life
M u lt i p l e Bi rths: Two-o r M o re­
Fo r the Genetic Price of O n e
• Wh ich human cha racterist ics are sig11 ificr111 t ly
influenced by heredity?
• What happens during the prenatal stages of
development?
• What are the th reats to the fetal e11 viro n 111e11l, and
what can be done about them?
Earliest Develop ment
Th e blueprin ts for creating a perso n are stored and com m u ­
n i c ated in o u r genes, the basic un i ts of genetic i n formati on.
T he rough ly 25, 000 h uma n genes are the b io­
logical equiva l ent of "softw are" that program s
70,000-1 00, 000 Genes
c.
e d eve I opmen t of all parts of t h e
-- t h e 1utur
body's "hardwa re."
Al l genes are com posed of speci fi c
nces of DNA ( deoxyr ibonuc leic acid)
seque
=
4 6 Chrom osomes
molecules. The genes a re arranged i n specific
locatio ns and in a spec ific o rder along 46
ch romo somes, rod-sh aped portions of DNA
=
23 Chromos ome Pairs
t hat are organized in 23 pairs. O nly sex cel ls­
the ova and the sperm-con tain half this
n u m ber, so that a child's mother and father
each provide one of the two ch rom osomes i n
One Hum an Cell
each o f t h e 2 3 pai rs. T h 4 6 c h romosomes
( i n 23 p a i rs ) i n the new zygote co n tain the
FIGURE 3- 1
gen e tic bl uep r i n t that wi l l guide cel l activity
for t he rest of t h e i n divid u a l 's l i fe ( Pe n n isi,
The Content s of a
I n ternational
S i n g l e Huma n C e l l
Human
2000;
Genome
uencing
Consort
i
u
m
,
200
1
;
see
Seq
Figu
re 3- 1 ).
At the momen t of
a
process
ca
l
Thro
l
ed
ugh
m
itosis,
which
conception, hu mans receive
for
the
repl
ica
t
acco
i
o
n
of
unts
of
most
types
70,000 to 1 00, 000 genes,
cel l s, nea rly a l l the cel l s of the body will con ­
contained on 4 6
chromosomes in 23 pairs.
tai n the same 4 6 ch romosom es a s t h e zygote.
46
D i zyg ot i c twi n s Twins who
are produced when two separate
ova are fert i l i zed by two separate
s perm at roughly the same time
PA RT O N E
Beg i n n i n g s
A l tho ugh it doesn't seem surpri i n g when d o g s a n d cat s g ive
birth to severa l offspring at one t i m e, i n h u ma ns, m ul t i p l e
b i rths a re cause fo r co m m en t . And t hey s h o u l d be: Less t h an
3% of a l l pregnancies p ro d u ce twi ns, a n d the o I ds a re eve n
sl i m mer for t h ree or more c h i l d re n .
Why do m u ltip le b i rths occ ur? S o m e occu r w h e n a c l us­
ter of cel ls i n th e ov u m spl i ts off with i n t h e fi rs t 2 weeks a fte r
fertilization. The res u l t i s two gen et i cal l y i d en t i cal zyg otes,
which a re called 111onozygotic beca use they co m e fro m th e
same original zygot e. Monozygotic t w i n a re tw i n s wh o are
genet ica l l y identica l . Any d i fferences 10 thei r fu t u re deve l op­
ment ca n be attributed o n l y to env i ro n me n t a l facto rs,
because genetically they a re exactly t h e sa me.
There is a secon d , a n d act ual l y m o re co m mo n , mec ha­
n ism that p rod uces m u l t i ple b i rths. In t hese case , t wo se p a­
rate ova are fert i l ized by two separate sperm a t ro u g h l y t he
same t i me. Tw i n s prod uced in t h i s fash i o n a rc k n ow n as
d i zygot i c t w i ns. Beca use they a re t h e res u l t of t wo separ a te
ovu m-sperm com b i nations, the y a re no more g e n et ical ly
s i m i l a r than t wo s i b l i n gs born a t d i ffere n t t i mes.
Of co u rse, n o t all m u l t i ple bi r t h s prod u ce o n l y t wo
babie . Tr i plets, q uadru plets, and even m o re b i r t h s a re pro ­
d uced by e i t h e r ( o r bo t h ) o f the m echa n i s m s t h at yield
twi ns. Thus, t r i plets may be some co m b i n at i o n of rn o n oz)1 got i , d i zygotic, o r t r izygo t i c .
A l t hough the chances of hav i ng a m u l t i p l e birth are typ­
ically sl i m , the odd rise con iderably when couples use fer­
t i l i t y d rugs lo i m p rove the probability of conceiving a child.
For exam ple, I in IO couple u ing fer t i l ity d ru gs have
d i zygotic twi ns, compared to an overal l figure of I in 86 for
a ucasian co u ples in the U n i ted tates. Older \\ Omen, too,
a rc more l ikely to have multiple birt hs, and multiple bi rths
arc also more co mmon in s me familie than they are in oth­
ers. The i ncreased use o f fe r t i l i t y d ru gs and rising average
age of m o t h ers g i v i n g b i rt h has mea n t that m u l t i p l e births
h ave i n c reased i n t h e last 25 yea rs ( see Figure 3 - 2 ; M a rt i n
c t a l . , 2005 ) .
There a rc also racial, ethn ic, a n d nat ional differences in
the ra te of m u l t i ple bi rths; this is probably due to inher ited
d i fferences in the l ikel i hood that more than one o, um will
be released at a time. One out of 70 frica n American cou­
ples have di zygotic birth , compared with the I out of 86 fig­
ure fo r \ 'h i te American couples ( Va ughan , McKay, &
Beh rman, I 979 ; \1\lood, 1 997 ) .
Mot hc.:rs carr y i n g mult iple children run a hi g her than
average risk of prem a t u re delivery and birth co mplications.
onsc q u e n t ly, these mothers must be particul a rl y concerned
abo u t t hei r p renatnl ca re.
"'
t'.
iii
40
30
. . . . . . . . . . . . . . . . . . . . . . .................... . . . . . . . . . . . . . . . . . . . . . . . . . . ..... .
0
0
0
aia.
"'C:
J
1 98 0
1 985
1 990
1 995
2000
2001
2002
2003
2001
2002
2003
Yea r
...
QJ
a. V,
QJ .c.
... ·t'.
-
200
1 50
. . ...... . . . ............. . . . . . . . . . . . . . . . . . . . . .
0
:? �
... 0
0 0
QJ 0
QJ 0
..c �
1 9 80
1 98 5
1 99 0
1 995
2000
Year
F I G U R E 3 - 2 R i s i n g M u ltiples
Multiple births have increased significantly over the last 25 years.
What are some of the reasons for this phenomenon?
(Source: Martin et al. . 2005)
Monozyg otic and dizygotic twins present opportu nities to learn about the relative
contributions of hered ity and situational factors. What kinds of things can
psychologists learn from studyi ng tw ins?
Bo y or Girl? Establishin g the Sex of the Child Recall
that there are 23 matched pairs of chromo ome . In 22 of these
pairs, each chromosome is imi. l ar to the other member of its
pair. The one exception is the 23rd pair, which is the one that
determine the sex of the child . ln females, the
23rd pair consi ts of two match i ng, relatively
large X-shaped chromosomes. appropriately
identified as XX. ln males, on the other hand,
the members of the p air are dissimilar. One
consists of an X-shaped chromosome , but the
other is a shorter, smaller Y-shaped chr m
s me. This pair is identified as }..'Y.
A we discussed earlier, each g amete
carrie one chromosome fro m each� of the
parent's 23 pairs of chromosome . Be au a
female's 23rd pair of chromosomes are both
Xs, an ovu m will always carry an X chromo­
some, no matter 1,-v hich chroma ·ome of the
23rd pair it get . A male' 23rd p air is , Y, -o
each sperm could carry eith r ·rn X o r a Y
c h romosome
I f the sperm con t ributes a n X ch ro m o­
some when it meets an o u m ( which,
re member, will always co n t r i b u te a n X
.,
c h romosom e ) , the child w i l l have a n X "'\:
Not only is the X chromosome
pa i r i n g on the 23rd chro mosoml:' and w i l l important in determining
b e a female. l f t h e sp erm o n t ribute a Y gender. but it is also the site of
ch ro mosome. the res ult will b an XY pai r ­ genes controlling other
ing a n d will be a male ( see Fi gure 3 - 3 ) .
aspects of development.
C vt. CI Jt) ter 3
The Start of Life: Genetics and Prenatal Development
47
Dominant trait The one trait
that i s expressed when two
compet i n g traits are present
Recessive tra it A trait within
an organism that is present, but
is not expressed
G e n oty p e The u n d e rlyi n g
c o m b in a t i o n of g e n e t ic materi a l
present (but n o t outward l y
v i s i b l e) i n an o rg anism
It is clear from this process that the
father's sperm determi11es the gender of the
child. This fact is leading to the development
of techniques that will allow parents to
increase the chances of specifying the gen ­
der of their child. In one new tecl1 11 ique,
lasers measure the DNA in sperm. By dis­
carding sp e rm that harbor the w1wanted sex
chromosome, the chances of having a child
of the desired sex increase dramatically
(Belkin, 1 999; Van Balen, 2005 ) .
O f course, procedures fo r choosing a
child's sex raise ethical and practical issues.
For example, in cultures that value one
gender over the other, might there be a
kind of gender discrim ination prior to
birth? Furthermore, a shortage of children
of the less preferred sex might ultimately
emerge. Many questions remain, then,
before sex selection becomes routine
( Sharma, 2008) .
The Basics of Genetics:
The M ixing and Matc h i n g
of Traits
What determined the color of your hair?
Why are you tall or short? What made you
susceptible to hay fever? And why do you
have so many freckles? To answer these
When an ovum and sperm meet at
the moment of fertilizati o n , the
questions, we need to consider the basic
ovum is certa in to provid e an X
mechan i sms involved in the way that the
chromosome, wh ereas the sperm
genes we i nherit from our parents trans­
will provide either an X or a Y
mit information.
chromoso me. Jf the sperm
Vie can start by examining the discov­
contributes its X chromosom e , the
eries of an Austrian monk, Gregor Mendel,
child will have an XX pairi ng on the
in the mid- l 800s. In a series of simple yet
23rd c h ro mosome and will be a girl .
convincing experiments, Mendel cross­
If the sperm contributes a Y
pollinated pea plants that always produced
chromosome, the resu lt will be an
yellow seeds with pea plants that always
XY pairing a n d will be a boy. Does
produced green seeds. The result was not,
this mean that g i rls a re more l i kely
as one might guess, a plant with a combi ­
to be conceived than are boys?
nation of yellow a n d green seeds. Instead,
all of the resulting plants had yellow seeds. At first it appeared
that the green-seeded plants had had no influence.
However, addi tional research on Mendel 's part proved
th is was n ot t r u e . He b red toge t h e r p la n t s fro m t h e n ew,
ye l l ow- seeded gen e r a t i o n that had res u l ted from h i s
o r i g i n a l cross - b reed i n g of the gre e n - seeded and ye llow­
seeded plants. The consistent result was a ratio of th ree­
fourths yel l ow seeds to one- fo u rth green seeds.
Why did th is 3-to-l ratio of yel low to green seeds appear
so consi stently? ft was Mendel's gen i us to provide an a nswer.
Based on h i s experiments w i th pea plan ts, he argued that
48
PA RT O N E
B eg i n n i ngs
P h e n otyp e An observable
t rai t : the trait that actually is seen
H o m ozyg ous I n heri t i n g fro m
parents s i m ilar genes for a given
trait
H eterozygous Inheriting from
parents d ifferent forms of a gene
for a given trait
genes fro m each p a re n t . I f the child receives similar genes, he
is said to be h o m ozygous for the trait. On the other h a n d , i f
t h e ch i l d receives d i ffere n t forms of t h e gene from h i s par­
ents, he is sa i d to be heterozygous. I n the case of heterozy­
gous alleles ( 13b ) , the d o m i n a n t characteristic, b rown eyes, is
exp ressed . However, if the child happens to receive a reces­
sive allele from each o f his paren ts, and therefore lacks a
dom i n a n t cha racteristic ( bb ) , he ,, ill display the recessive
characterist ic, such as blue eyes.
Trans m i s s i o n of G e n eti c I nfo rmation
Gregor Mendel's pio neeri ng experiments o n
pea p l a n t s provided t h e fou n d ation for the
study of genetics.
wh en two com pet ing t ra i ts, such as a g reen or yel lo w c olor ­
ing o f se eds, were both p rese n t , o n l y o n e co u ld b e e x pre ssed
( di splayed) . The one that was exp ressed was ca l l ed a
dom i n a n t tr a i t . Meanwh ile, t h e o t h e r t ra i t rem a i n e d p res ­
e n t i n the orga n i s m , a l though n o t exp ressed . Th i s was
called a rec essiv e t ra i t . In the case of M e n d e l ' s o rig i n a l pea
pla nts, t he offsprin g p l a n ts received gen etic i n form ation
fro m bot h the gree n - seed ed a n d t h e yel l ow-seede d pa re n ts.
However, th e yellow trait was d o m i n a n t , a n d conseq ue n tly
the recessive green t ra i t d i d not asse r t itself.
Ke e p i n mi nd, ho we v er, that genet ic material rela t i ng t ?
bot h pare nt plants is present i n the o ffs p r i n g, even th o ugh it
cannot be seen . The genetic i n formatio n is kno w n as the
� rga n i s m's genotype . A genotype is the underlying comb i n a­
tion of genet ic material p resent ( but outwardl y i n visib le) in a n
or� ani sm. I n contrast, a phenotype is t h e o bservable tra i t , the
tr ait that actu ally is seen. Al though th e offspri n g of the yel low­
seede d an d green-seeded pea plants all h ave yel l o w see ds ( i.e . ,
th ey h ave a yell ow-seeded phenotype ) , the ge notyp e consis ts
of ge netic i n formation relating to both paren ts.
And wha t is the nature of the i n fo rmat i o n in t he geno ­
ty pe? To answer that question, let's turn fro m peas to people .
I n fact, the princi ples are the sa me not only fo r pla nts and
human s but also for the majorit y of spec i es.
Recall that parents transmit genetic i n formatio n to thei r
offspring v i a t h e chromosomes they co n t r i b u te t h ro ugh the
gam ete they provide d u ring fer t i l i za t i o n . Some o f t h e genes
form pai rs called alleles, ge nes govern i ng traits t h a t may take
alternate forms, such as h a i r or eye color. Fo r exa m ple,
b rown eye color is a dom inan t t ra i t ( R ) ; b l u e eyes a re reces­
sive ( b ) . A ch i ld 's a l l e l e may conta i n s i m i l a r o r d i ss i m i l ar
We c a n see t h i s p rocess at work in h u m ans by considering
the t ransmission of pl1c11ylketo 1 1 1 1 ria (PKU) , an inherited dis­
order in which a child is unable to make use o f phenylala­
n i ne, an esse n t i a l a m i n o acid p resent in proteins found in
m i l k and other foods. If left untreated, PKU allows phe­
nylal a n i n e to build up to toxic levels, causing brain damage
and mental retardat i o n .
PKU is p rod uced b y a si ngle allele, or p a i r of genes. As
show n in Figure 3-4, we can label each gene of the pair with
a P if it carries a d o m i n a n t gene, which causes the normal
production of phenylala n i ne, or a p if it carries the recessive
gene that p ro d u ces PKU. In cases in which neither parent is a
P K U ca rrier, both the mother's and the father's pairs of
genes are the domi nant form, symbolized as PP. Conse­
quently, no matter wh ich member of the pa i r is cont ributed
by the mother and father, the resulting pair of genes in the
child will be PP, a n d the child will not have PKU.
However, consider what happens if one of the parents
has a recessive p gene. In this case , whi ch we can we symbol­
i ze as Pp, the parent will not have PKU, because the normal P
ge ne is dominant. But the recessive gene can be passed down
to the child. This is not so bad : If the child has only one reces­
sive gene, it will not suffer from PKU. But what if both par­
en ts carry a recessive p gene? In this case, although neither
parent has the disorder, it is po ssible for the child to receive a
recessive gene from both parents. The child's geno type for
PK U then will be pp, and he or she will have the disorder.
Rem ember, though, that even children whose parents
both have the recess ive gene for PKU have only a 25%
chance of inheriting the disorder. Due to the laws of proba­
bility, 25% of ch ildren with Pp parents will receive the dom­
i n ant gene from each parent (these children's genotype
would be PP) , and 50% will receive the dominant gene fro m
one parent and the recessive gene from the other ( their
geno types wo uld be e ither Pp or pP) . Only the unlucky 25%
wh o receive the recessive gene from each parent and have the
genotype pp will suffer fr o m PKU.
Polyg enic Traits The transmission of PKU is a good way
to ill ust ra te the basic princi p les of how genetic i nformation
passes from parent to child, altho ugh the case o f PKU is sim­
pler than most cases of ge netic transm ission. Relativel y few
traits are governed by a si ngle p air of genes. I nstead, most
Normal
Carries recessive Ca rries recess ive
PKU gene
PKU gene
Ca rrier
C a r r i e r Affl i cted
with P K U
R e s u l t : One in f o u r ch i l d ren w i l l i n herit two
d o m i na nt genes a n d w i l l not have P K U ; two
in fou r w i l l i n h e rit one recessive gene and not
be affl icted with P K U but w i l l carry the
recessive gene: a n d o n e i n four will have P K U
F I G U R E 3-4 PKU Probabi l ities
PKU, a d isease that causes brai n damage and mental retardation, is pro d u ced by a
single pair of genes inherited from o ne's mother and father. If neither parent carries
a gene for the disease (a), a child can not develop PKU . Even if o n e parent carries
the recessive gene, but the other doesn 't (b), the c h i ld cannot i nherit the disease.
However, if both parents carry the recessive gene (c), there is a one i n fou r c h ance
that the child will have P K U .
Ch/;l,l'.)ter 3
The Start of Life : Genetics and P renatal Development
49
Polygenic inheritan ce
Inheritance in whi ch a
combi nation of multiple gene
pairs is respo nsible for the
production of a particular trait
X- lin ked genes Genes that
are considered recessive and
l ocated only on the X
chromosome
B e ha v i o ra l g e n et i c s
T h e study of t h e effec ts of
heredity on b ehavior
FIGURE 3-5
I n h erit i n g H e m o p h i l i a
Hemoph i l i a , a blood - c lotting disorder.
has been an inherited problem
t h ro u g h o u t the royal families of Europe,
as i l l ustrated by the descendants of
Queen Victoria of Britain .
(Source: Adapted from Kimbal l , 1 983)
traits are the result of polygenic inheritance. In pol yge n ic i n ­
heritance, a combi nation o f multiple ge n e pa i rs is res p o n s i ­
ble for the production o f a particular trait.
Furth erm ore, s o m e genes come in several alternate
forms, a n d still o thers act to modify t h e way that p a r t i c u l a r
genetic traits ( p roduced by other a l l e l e s ) a re disp layed .
Genes also vary i n terms o f t h e i r reaction ra nge, the pote n ­
t i a l degree o f variab i l i t y i n the actual express i o n o f a t r a i t
due to environ m e n t a l c o n d i t i o n s . A n d
some traits, such as blood typ e , a re p ro ­
du ced by genes i n wh ich nei ther member
o f a pair o f genes c a n b e class i fi e d as
p u rely d o m i n a n t or recessive . I nstead ,
the t rait i s exp ressed i n ter ms of a c o m b i ­
nation of t h e two genes-s uch a s t y p e A B
blood.
A number o f recessive genes, called X­
linked genes, are located only on the X
ch romosome. Recall that i n females, the
23rd pair of chromosom es is an XX pai r,
whereas in males it is an XY pair. One result
is that m ales have a higher risk for a variety
of X-linked disorders, because males lack a
seco nd X chromosome that can co unteract
the genetic information that prod uces the
Esti mated percentage o f each
disorder. For example, ma les are sign i fi ­
crea ture' s tota l genes fou n d i n
cantly more apt to have red-green color
hum ans are ind icate d by th e
blind ness, a disorder produced by a set of
dotted l i ne .
genes on the X chromosome .
FIGURE 3-6
S i m i l arly,
the
blood
d i s o rd e r
U n i q uel y H u m a n ?
hemoph ilia i s p ro d u ced by X - l i n ked
H umans h ave about 2 5 , 000
ge n e s . H e m o p h i l i a h a s been a rec u r re n t
gene s, m aking them n ot muc h
p ro b l em i n t h e roy a l fa m i l i es o f E u ro p e ,
more genetic ally complex than
a s i l l u s t r a t e d i n F i g u re 3 - 5 , w h i ch s h ows
some prim itive specie s.
t h e i n h e r i ta n ce o f h e m o p h i l i a in the
(Source: Cetera Genomics :
desc
e n d a n t s o f Q u ee n V i c t o r i a of G r e a t
International Human Genome
Sequencing Consorti u m . 2001 )
B r i ta i n .
50
PART O N E
Beg i n n i n gs
The H u m an G e n o m e and B e h avi oral
Gen etics : C rac k i n g the Gene t i c Cod e
Men d e l 's a c h i eve m e n t s i n recog n i z i n g t h e ba s i c s o f g e n et i c
t r a n s m i ssio n o f t ra i t s were t ra i l b l a z i n g . H owe v e r, t h ey m a r k
o n l y th e begi n n i n g o f o u r u n d e r s t a n d i n g o f t h e w a y s t h ose
pa rt i c u l a r s o rt s of c h a r a c t e r i s t i c s a rc passed on fro m o n e
gen era t i o n to th e next. Th e most recen t m i l e st o n e i n u n d er­
sta nd i ng ge n et i c s was re a c h e d i n ea r l y 200 I , w h e n m o l ec u la r_
ge n e t i cists s u cc eede d i n m a p p i n g t h e s p ec i fi c seq u e n ce o t
ge nes o n eac h c h ro mosome. Th i s acco m p l i s h m e n t sta n ds as
? n e of t h e mo st i m po r ta n t m o m e n t s i n t h e h i s t o r y of genet­
JCS, a n d , fo r t h a t m a t t e r, all o f b i o l ogy ( I n t e rn a t i o n a l H u m an
Geno m e Seq u e n c i n g Co n so rt i u m , 200 1 ) .
Al rea dy, t he mappi ng o f the gene seq uence has pro vid ed
impo rtant a dvan ces in o u r u n derstand i ng of genet ics. Fo r
i n s tan c e, the n u mber of h u m a n ge n es, long though t to be
1 00, 00 0, has be en revi sed downwa rd to 2 5 ,000- not m any
� ore than is found i n organisms that a re fa r l ess co m p l ex ( s ee
Fi gu re 3 - 6 ) . Fur the rmore, scientists h ave d iscovered t ha t 99 . 9%
of the gene s equ ence is shared by a l l h u m ans. I n sho rt, t his
means that we humans a re far m o re s i m i l ar to o ne a not her th a n
we are d i fferen t . It also indicates that m a ny of the d i ffe ren ces
that see m i ngly separate peopl e-such as race-a re , l iter a lly,
o nl y. ski n d e ep . Human genome m a p p i n g w i l l al so help iden t i fy
particula r d iso rders to wh ich a give n i n d i v i d ual is s u scep ti ble
( Gee, 2004; Delisi & Fleisch haker, 2007; Gupta & State, 2 007) .
The m a p p i n g of t h e h u m a n gen e s e q u ence i s s u p p o r t i n g
the fi e l d of behavio ral gen e t i cs. A s t h e n a m e i m p l i es ,
behavi oral genetic s stud ies the effects o f h e red i t y o n psy­
chologica l c h a racteristic s. R a t h e r t h a n s i m p l y exa m i n e sta ­
b l e , u n c h a n g i n g characte ristics su c h as h a i r o r e ye col o r,
behav i o r a l ge net icists take a b ro ader a p proach a n d con s i d e r
h o w o u r pers o n a l i t y a n d behav i o r a l h a b i t s a re a ffect ed b y
ge netic factors. Perso nal ity tra i ts s uch as s h yness o r sociabi l i t y,
mood i n ss a n d assertiveness a re a m o n g t h e a reas b e i n g
s t u d i e d . O t h e r b e h a v i o r a l ge n e t i c i s t s s t u d y p s yc h o l o g i c a l
The Genetic Basis of Sel ected Behavi oral Disorders and Traits
Behavio ral Trait
Current Ideas o f Genetic Basis
H u ntington's disease
Huntington gene has been identified.
Early-onset (fa m i l ial) Alzhei mer's d i sease
Three distinct genes have been id entified .
Frag i l e X mental ret ard ation
Two genes have been identified.
Late-on set Alzheimer's di sease
One set of genes has been associated with increased risk.
Atte nti on -deficit/hyperact ivity d i sorder
Three locations related to the genetics invo lved with the
neurotransmitter dopamine may contribute.
--------
Dyslexia
Relationships to two locations, on chromosomes 6 and 1 5 ,
have been suggested.
Schizophrenia
There is no consensus, but l inks to nume rous chromosomes,
including 1 , 5 , 6 , 1 0 , 1 3 , 1 5 , and 22 have been reported .
(Source:
Adapted from McGullin, Riley, & Plomin, 200 1 )
d i s o rders such a s d e p ressi o n , attention- deficit/hyperac t i vity
d i so rder, a n d sch izoph ren i a , looking fo r poss ibl ge netic
l i n ks ( Baker, Mazzeo, & Kendler, 2007; DeYo u ng, Quilty, &
Pe t e r s o n , 2007; Haeffel et al., 2008; see Ta ble 3 - 1 ) .
The p rom ise o f behavioral ge netics i s substa ntial. For one
t h i ng, resea rchers worki ng within the field are gai n i n g a better
understa n d i n g o f the specifics of the ge netic code t h at u nder­
lie h u m a n behavior and development. Eve n more important,
researchers are seeki ng to i dent i fy how genetic defects may be
remed ied ( P l o m i n & Rutte r, 1 998; Peltonen & McKusick,
200 1 ) . To u n d e rstand how that goal might be reached, we
need to co nsider the ways in which genetic facto rs, which nor­
mally cause develop m ent to p roceed so smoothly, may falter.
Alternat ive ly, certain environmental facto rs, such as
exposure to X- rays o r even to h ighly polluted air, may
uce a malformation of g e netic mater ial ( see Figure 3 - 7 ) .
prod
V
\,hen such dama g ed g enes are p assed o n to a child, the
I n herited and Genetic Disorders:
When Deve l o p ment Goes Awry
P K U is j u st one of several disorders that may be i n herited.
Like a b o m b t h a t i s harm l ess u n t i l its fuse is lit, a recessive
gene res p on s ible for a d iso rder may be passed on unknow­
i n gly f r o m o n e generation to the next, revealing i tself only
when, by cha nce, i t is p a i red with another recessive gene . I t is
only when two recessive genes come toget her l i ke a match
a n d a fuse that the gene w i l l exp ress itself and a child w i l l
i n h e r i t the ge n e t i c disorder.
But t h e re is another reason that genes can be a source of
conce rn : I n so m e cases, genes beco me physically d a m a g ed.
For i n s tance, genes may b reak dow n due to wea r-an d - tea r or
to cha nce eve n ts o c cu r r i ng d ur i n g the cell-division p ro cesses
of meiosis a n d m i tosis. And someti mes, for no known rea­
so n , genes s p o n t a n eously cha nge t h e i r for m , a p rocess called
spontn neous m 1 1 t11tion .
F I G U R E 3- 7 I nhaled Ai r and Genetic M utations
Inhalation of unhealthy, polluted air may lead to mutations in genetic material in sperm .
These mutations may be passed on, damaging the fetus and affecting future generations.
(Source: Based on Samet. DeMarini. & Malling. 2004 . p. 97 1 )
C hlil.11 te r 3
The Start of Life: Genetics and Prenatal Development
Sl
Down syndrome A disorder
produced by the presence of an
extra c h romosome on the 2 1 st
pair; once referred to as
mongolism
Fra g i l e X syndrome
A disorder prod uced by injury to
a gene on the X chromosome,
producing mild to moderate
mental retardation
Sickle- c e l l a ne m i a A blood
d isorder that g ets its name from
the shape of the red blood cells in
those who have it
Tay-Sa chs disease A d i s ord e r
t h a t produces b l i n d ness a n d
m u scle degen eration p r i o r t o
death; there is no treatment
results can be disastrous m terms o f fu t u re p hysical a n d
cogn itive development (Samet, DeMari n i , & M a l l i ng,
2004 ) .
In addition t o P KU, which occurs once i n 1 0 ,000 to
20,000 births, other inheri ted a n d gen etic d isorders i n c l u d e :
• Down syndrome. As w e noted earlier, m o s t people have 4 6
chromosomes, arranged in 23 pairs. O n e exception is i n d i ­
viduals with Down syndrome, a disorqer e.!:od uced b y t h e
presence o f an extra chromosome on t h e 2 1 st a i r. Once re­
ferred to as mongolism';- Down syn rome is the ;;;��re­
quen t cause of mental retardation. It occurs in abo u t 1 o u t
of 5 0 0 births, although the risk i s m uch grea ter i n mothers
who a re unusually yo ung or old ( Crane & Morris, 2006 ) .
• Fragile X syndrome. Fra gile X syndrome occu rs when a
p articular gene is injured o n the X chromosome. The re­
sult is mild to moderate mental retard a t i o n .
• Sickle-cell anemia. Around one-tenth o f t h e African Ameri­
can population carries genes that produce sickle-cell anemia,
and 1 African American i n 400 actually has the djsease. SickJe­
cell anemia is a blood rusorder that gets its name from the
shape of the red blood cells in those who have it. Symptoms
include poor appetite, stunted growth , swollen stomach, and
yellowish eyes. People afflicted with the most severe form of
the disease rarely live beyond childhood. However, for those
�th l � ss severe cases, medical advances have produced signif­
tcant mcreases in life expectancy.
• Tay-Sachs disease. Occurring mainly in Jews of eastern
E � ropean ancestry and in French Canadians, Tay-Sachs
disease usually causes death before its victims reach school
a g_e. There is n o treatment fo r the disorder, which produces
blmdness and muscle degeneration prior to death .
• Klinefelter's syndrome. One male out of every 400 is born
with Klinefelter's syndrome, an abnormality resulting from
the presence of an extra X chromosome. The resulting XXY
complement p roduces underdeveloped genitals, extreme
heigh t, and enlarged breasts. Klinefelter's syndrome is one of
a number of gen eti c abnormalities that result from receiving
the improper number of sex chromosomes. For instance,
K l i nefe lte r's syn d ro m e
A di sorder res u l t i n g from t11e
presence of an extra X
chromosome that produces
underdeveloped genitals,
extreme height, a n d enlarged
breasts
there a re disorders p rod uced b y a n ext ra Y c h ro m osome
( XYY ) , a m issi n g second c h romosome ( ca l l ed Tu rner syn­
drome; XO), and th ree X c h ro m osomes ( X X X ) . Su c h d isor­
ders are typically c h a racterized by p robl e m s rel a 1 i ng to
sexual characterist ics and by i n tellec t u a l defi c i t s ( Ke. ler,
2007; J. Ross, Stefanatos, & Roel tge n , 2007 ) .
J t i s i m porta n t t o keep i n m i n d t h a t t h e m re fa c t a d i s ­
o rd e r has ge n e t i c ro ots d oes n o t m e a n t h a t e nv i ro n m e n t a l
factors do not also play a role ( Mo l d i n & Go t t es m a n , I 997) .
Co nsider, fo r i ns t a n ce, sickl e-cel l a n e m i a , wh ich p r i m a r i l y
a ffl icts people of A fr i c a n desce n t . Because t h e d i se.ise ca n b e
fatal i n ch i l d h ood , we'd ex pect t h a t t h ose w h o s u ffc..: r fro m i t
wo uld be u n l ikely to l i ve long e n o u g h t o p a s s i t o n . A n d t h i s
does seem to be t ru e , a t l east i n t h e U n i ted S t a tes: C o m p a re d
w i t h pa rts of West A fr i c a , the i n c i dence i n t h e U n i t ed S t a t es i s
much l owe r.
B u t why s h o u l d n ' t t h e i n c i d e n c e o f s i c k l e - c e l l a n e m i a
a l s o be grad u a l l y red u ced fo r peo p l e i n We s t A fr i ca ? T h is
q u e s t i o n p roved p u zzl i n g fo r m a ny yea r s , u n t i l s c i e n t i s t s
d e t e r m i n ed t h a t c a r r y i n g t h e s i c k l e - c e l l ge n e ra i ses i m m u ­
n i t y t o m a l a r i a , wh i c h i s a c o m m o n d i sease i n Wes t A fr ica
( A l l i s o n , 1 9 54 ) . T h i s h e i g h te n e d i m m u n i t y m e a n s t h a t
p e o p l e w i t h th e s i c k l e - c e l l gene h a ve a gen e t i c a d va n t a ge
( i n terms o f res i s t a n ce to m a l a r i a ) t h a t o ffs e t s , to s o m e
d eg ree, t h e d i sadva n t age o f b e i n g a c a r r i e r o f t h e s i ck l e ­
c e l l gene.
The lesson o f s i c k l e - c e l l anem i a is t h a t ge n e t i c fa c to rs
are i n te r t w i n ed w i t h e n v i ro n m e n t a l co n s i d e ra t i o n s a n d
c a n ' t be l o oked at i n i so l a t i o n . F u r t h e r m o re , we n eed t o
r e m e m b e r that a l t h o ugh we've been fo c u s i n g o n i n h e r i ted
fac tors t h a t can go aw r y, i n the vast m aj o r i t y o f c a s es t h e
genetic mech a n i s m s w i t h w h i c h we a re endowed wo rk
q u i te wel l . Overall , 95% o f ch i l d re n born in the U n i ted
S t a te s are healthy a n d n o r m a l . For t h e s o m e 2 5 0 ,00 0 wh o
a re bo rn w i th some s o r t o f physical o r m e n t a l d i so rd e r,
a p p ro p r i a t e i n terve n t i o n o ften c a n h e l p t reat a n d , i n so m e
cases, c u re t h e p ro b l em .
Moreover, due to advances i n beh avioral gene t i cs, gen et ic
.
diffic ulties increasingly can be fo recast, anticipated , a nd
plan ned for before a chi l d 's birth, enabling parents to t ake
steps before the child is b orn to red uce the severity of ce r ta i n
geneti c conditions. I n fact, as scientists' kn owledge rega rd i ng
the specifi c locatio n of partic u l a r genes expa nds, p red i c t i on s
of what the genetic fu ture may hold a re beco m i ng i n c reasi n gly
exact, as we d iscuss n ext ( Pl o m i n & Rutter, 1 99 8 ) .
Genetic Cou nsel i n g : P red i cti ng
the Futu re From the Genes
of the Present
Sickle-cell anemia, named for t h e presence of
m isshapen red blood cells, is carried in the genes of
1 in 1 0 African Americans.
52
PA RT O N E
Beg i n n i ngs
If you knew that yo ur mother a nd grand mother h a d d ied of
Hunti ngton's disease-a devastat i n g, a lways fatal i n h erited d is­
order marked by t remors a n d i n tellect ual deterioratio n-to
Genetic c o u n se l i n g
The discipline that focuses on
helpin g peo ple deal with i ssues
relating to inherited d isorders
U ltra s o u n d sonography
A process i n which high­
freq uency sound waves scan the
mother's womb to prod uce an
image of the un born baby,
whose size and shape can then
be assessed
whom could you t u rn t o lea rn your own cha nces of comin g
down w i t h t h e d isease? The best person to seek wo uld be a
genet ic co u nselor, a mt:mber of a fiel d t h at, u n t i l a few dec ad es
ago, was nonexis � cn 1 ., Ge n e � ic co u ns e l in g focuse on hel p i ng
_
people deal w i t h issues rela t 1 ng to i n h eri ted disor er .
Gep e t i c co u n selo rs use a variety of d a ta-i n t h �vo rk.
�or i nst,mce, co u p l es co n te m p l at i ng hav i ng a c h i ld may see '
to dete r m i n e t h e r i sks i nvol ved i n a fu t u re p regriancy. n
such a case, a cou nselor w i l l t a ke a t h o ro ugh fa m i l y h i sto ry,
seek i n g a ny fa m i l i a l i n cide nce of b i rth defects t h a t m i g h t
i n d i c a t e a p a t t e rn o f recess ive o r X - l i n ked ge nes. I n addition ,
th e counsel o r w i l l assess fac t o rs such as the age of the
m o ther a n d fa t h e r and a ny prev io us a b n o r m a l i t i es i n o t h er
c h i l d ren t h ey m a y have a l ready had ( Resta et a l . , 2006 ) .
Ty p i c a l l y, ge n e t i c counselors sugges t a t h o rough physi­
cal exa m i n a t i o n . Such a n exam may i den t i fy physical abnor­
m a l i ties that p o t e n t i a l parents may h ave and n o t be aware of.
I n add i t i o n , s a m ples o f b l o o d , ski n , a n d u r i n e m ay be used to
iso l a te a n d ex a m i n e spec i fic c h ro m oso mes. Possible ge netic
defects, such as t h e p rese nce o f an extra sex c h ro m osome,
Chorionic v i l l u s sampl i n g
(CVS) A test u s e d t o find
genetic defects that involves
taking samples of hairlike
material that su rrou n d s the
embryo
can be identi fied by assem b l i n g a ka ryotype, a chart con­
t a i n i ng e n l a rged photos of each of t h e c h romosomes.
A va r i ety of tech niques can be used
to assess the health o f an unborn child if a woman is al­
ready pregna n t ( see Tab l e 3 - 2 fo r a list of currently avail­
able tests ) . The earliest test is a fi rst- t,·irnester screen , ,,v hich
comb i n es a blood test and u l t rasound sonography in the
I I th to 1 3 th week of pregnancy. In ultrasourul.s_o.n_Qgia­
phy, high-frequency sound waves oo mbard the mother's
womb. These waves p roduce a rather i ndistinct, but use­
ful , image of the u nborn baby, whose size and shape can
then be assessed. Re peated use o f ultraso u n d sonography
can reveal devel opmental patterns. Altho ugh the accura cy
of blood tests and ultraso u n d in iden t ifying abnormal ities
i s not high early in p regnan cy, it becomes m o re accurate
later o n .
A more invasive tes t, chorionic villus sampling
(CVS) , c a n be employed in the 1 1 t h to 13th week if blood
tests and ultrasound have identi fied a potential p roblem .
Prenatal Testi ng
Fetal Development M o n itori ng Tech n i q u es
Technique
Description
A m n iocentesis
Done between the 1 5th and 20th week of pregnan cy, this proced u re examines a sample of the
amniotic fl u i d , which contains fetal cel ls. Recommended if either parent carries Tay- Sach s ,
s p i n a bifida, sickle-cell , Down syndrome, muscu l ar dystrophy, or Rh d i sease .
Chorionic villus sam p l i n g (CVS)
Done at 8 to 1 1 weeks, either transabdominally or transcervical ly, depen d i n g on where the
placenta is located . Involves inserting a need le (abdominally) o r a catheter (cervically) i nto the
su bstance of the placenta but staying outside the amniotic sac and removing 1 O to
1 5 m i l l igrams of tissue. This tissue is manually cleaned of maternal uteri ne tissue and then
grown i n culture, and a karyotype is made, as with amniocentes i s .
Em bryoscopy
Exami nes the embryo or fetus during the fi rst 1 2 weeks of preg nancy by means of a
fiber-optic endoscope inserted through the cervix. Can be performed as early as week 5 .
Access t o the fetal circu lation may be obtai ned through t h e i n strument , a n d d i rect
visualization of the embryo permits the diagnosis of malformations.
Fetal blood sampling (FBS)
Performed after 1 8 weeks of preg nancy by collecting a small amount of blood from the
umbi lical cord for test i n g . Used to detect Down syn d rome and most other c h romosome
abnormalities in the fetuses of coup les who are at increased ris k of having an affected c h i l d .
Many other d iseases c a n be di agnosed using t h i s technique.
Sonoem bryology
Used to detect abnormal ities i n the fi rst trimester of pregnancy. I nvolves h i g h-frequency
transvag i nal probes and dig ital image processing. I n combination with u ltrasound , can detect
more than 80% of all malformations during the second trimester.
Sonogram
Uses ultrasound to produce a visual image of the uterus, fetus, and placenta.
U ltrasound sonography
Uses very h igh-frequency sound waves to detect structural abnormal ities or m u ltiple
preg nancies, measure fetal growth, judge gestational age, and eval u ate uteri n e abnorm alities.
Also used as an adjunct to other proced ures such as amniocentesi s .
The Start of Life: Genet ics and Prenatal Developm ent
53
Amn iocentesis The process
of identifying genetic defects by
exam i n i n g a small samp l e of
fetal cells drawn by a needle
inserted i nto the amniotic fl u i d
surrounding t h e un born fetus
In amn iocentesis, a sample of fetal
cells is withdrawn from the amn iotic
sac and used to identify a number
of ge netic defects.
Th is proced u re i nvolves inserting a
thin needle i n to the fetus a n d
extracting s m a l l samples of hairl i ke
the
material
that
surro u n ds
em bryo. The test can be done
between the 8th and 1 1 th week o f
preg nancy. However, it produces a
risk of m isca rriage of I i n I 00 to l
in 200; because of the r i sk, its use is
relatively i n frequent.
In amniocentesis, a small sample of fetal cells is drawn by
a tiny needle i nserted into the amn iotic fluid surro u n d i n g the
unborn fetus. Carried out 15 to 20 weeks into the pregnancy,
amniocentesis allows the a nalysis of the fetal cell s that can
identify a variety of genetic defects with nearly 1 0 0% accuracy.
In addition, this test can determine the sex of the ch i l d .
Although there is always a da nger t o t h e fetus in an invasive
pro cedure such as amniocentesis, it is generally safe.
After the various tests are complete and all possible i n for­
mation is available, the couple will meet with the genetic
counselor again. Typically, counsel ors avo id giving specific
recommendations. I nstead, they lay out th e facts and present
various options, ranging from doing nothing to taki ng m ore
drastic steps, such as terminating the pregnancy th rough
abortion. Ultimately, it is th e parents who m ust dec ide what
course of acti on to follow.
S c reen i n g fo r Future Prob l e m s The newest role fo r ge­
n e t i c co u n sel ors i n vo lves testing people to i d e n t i fy whether
they th emselves, rath e r t h a n th eir ch i l d re n , are susceptible
to fu t u re d i s o rders because of gen etic a b n o r m ali ties. For
i n stance, H u n t ington's d i sease typ i cally does not appear
until peop le reach t h ei r 40s. However, g e n et i c testing can
i d e n t i fy m uch e a rlier whether a p e rson car ries the fl awed
ge n e t h a t produces H u n t i n gton's d isease. Pres u m a b l y, peo­
p l e's know l e d ge t h a t t h ey ca rry t h e ge ne can help them p re­
p a re t h e m selves fo r t h e fu t u re ( E n s e n a uer, M i chels, &
Re i n ke, 2 0 0 5 ; C i n a & Fell m a n n , 2006 ) .
I n a d d i t i o n t o H u n tingto n's d isease, m o re t h a n 1 , 000
d i sorders can be pred icted o n the basis of ge n e t i c test i n g
( see Ta b l e 3 - 3 ) . A l t h o ugh s u c h test i n g m a y b r i n g wel co m e
re l i ef fr o m fut u re worr i es-i f t h e resu l ts a re n egative­
p o s i t i ve res u l t s may p ro d uce j u s t t h e o p p o s i te effec t . I n
fa c t , ge n e t i c test i n g rai ses d i ffi c u l t p ract i c a l a n d e t h i c a l
q uesti o n s ( Jo h a n n e s, 2 00 3 ; Two m ey, 2006 ) .
54
PA RT O N E
B eg i n n i n g s
Suppose, fo r i n s t a nce,
From
wo m a n who t h o u gh t she w a s
a h e a l t h - c a re
s u s ce p t i b l e to H u n t i n g t o n 's
p ro v i d e r's
d i sease was t e s t e d i n h e r
perspective:
2 0 s a n d fo u n d t h a t s h e
1/1/ ·11 ;1r0 ·I·r · ,, , - ,. , , ' · 11
d i d n o t ca r r y t h e d e fe c ­
[H1(1 pi 11l(J<, 'I I r ii 1j I• •Si l 11 lS
t i ve gene. Obviously, s h e
il lrit SI r w1 1111 f t • ,; SSL(; Oi
wo u l d experience t remen­
\:J r ,lir. C°l llT lc-• · IIT U ' I , 1qi1t II
d o u s rel i e f. But s u p pose
·01 1 101 11 ne� I )( , t,r 11.·1Is , , · o
kl l(l'II , 11 l( ',1( I ,· .i ilf r " ' , ll Od
she fo u n d t h a t she d i d
po. ,•,,t)lr ur;r 1t · i 1 [ 11 1 111ked
carry t h e flawed ge ne a n d
rl1sorclc � 11 1,li · 1 H<Jl1
w a s t h ere fo re go i n g to gel
i l lir y, Jt r r I 11k f ,r
the d i sease. In t h i s case, she
yo, 11 s , ,li?
m ight wel l expe r i e n ce depres­
sion a n d remorse. I n fa c t , some
studies show that I 0% o f people who
find they h ave the fl awed gene that leads to H u n t i ngto n's dis­
ease never recove r fu l ly o n an e m o t i o n a l level ( G roo p m a n ,
1 9 9 8 ; Wa h l i n , 2007 ) .
Clearly, ge n e t i c test i n g i s a co m p l ica ted i ssu e . I t rarely
provides a s i m p l e yes or no a nswer as to w h e t h e r a n i n d i v i d ­
u a l w i l l be s usceptible to a d iso rder. I n stead , t y p i c a l l y i t p res­
ents a ra nge of p robab i l i t i es. I n s o m e cases, the l i kel i h ood of
actually beco m i n g i l l depe n d s o n the type o f e nv i ro n m e n tal
st ressors to which a pers o n i s exposed . Perso n a l d i ffe rences
also affect a given pe rso n's suscep t i b i l i t y to a d i so rd e r ( Pate­
naude, Guttmach er, & Col l i n s, 200 2 ; B o n ke et a l . , 200 5 ) .
A s o u r u n d e rs t a n d i n g o f gen et ics co n t i n ues t o grow,
resea rchers a n d m e d i c a l p ra c t i t i o n e rs h a ve m oved beyo n d
te s t i n g a n d counsel i n g to ac tively wo r k i n g t o c h a n ge flawed
genes. The possi b i l i t ies for ge n e t i c i n terven t i o n a n d m a n i p u ­
l a t i o n increas i ngl y b o rder o n wh a t o n ce w a s sc i e n ce fi c­
t i o n-as we consider in the Fro m Resea rch t o Prnct ice box on
page 5 6 about p re i rn p l a n ta t i o n gen e t i c d i a g n o s i s .
J
. R E V I E W ..J :
Some Cu rrently Available DNA-Based Genetic Tests
Disease
Description
Ad u l t pol ycystic kid ney disease
Kidney failure and l iver d isease
Alpha- 1 -antitrypsin deficiency
Em physema and liver disease
Alzheimer's d i sease
Late-on set variety of senile dementia
Amyotrophic lateral scl erosis (Lou Gehrig's disease)
Progressive motor function loss leading to paralysis and death
Ataxia tel angiectasia
Breast and ovarian cancer (inhe rited)
Progressive brain disorder resulting in loss of m uscle contro l and cancers
Charcot-M arie-Tooth
Congenital adrenal hyperplasia
Cystic fibrosis
Early-onset tumors of breasts and ovaries
Loss of feeling in ends of limbs
Hormone deficiency; ambig uous genitalia and male pseudohermaphrodrtism
Thick mucus acc u m u l ations in lu ngs and chro n i c infections in lungs
and pancreas
Duchenne muscular dystrophy (Becker
muscular dystrophy)
Severe to m i ld muscle wast i ng , deterioratio n , weakness
Dystonia
Factor V-Le iden
M u scle rigidity, repet itive twisting movem ents
Fanconi anemia, gro u p
Fragile X synd rom e
Gaucher d isease
Hemophilia A and B
Hered itary non polyposis colon cancer a
Huntingto n 's disease
Myotonic dystro phy
B lood-clotting disorder
Anemia, leukemia, skeletal deformities
Mental retardation
Enlarged liver and spleen, bone degenerat ion
Bleeding d isorders
Early-onset tumors of colon and someti mes other organs
Prog ressive neurological degeneration, usually beg inning i n m idl ife
Progressive muscle weakness
N e urofi bromatosis, ty pe 1
Multiple benign nervous system tumors that can be disfig u ring: cancers
Phenylkelo n uria
Progressive mental retardation due to missing enzyme; correctable by diet
Prader Wi l l i/An gelman syndromes
Sickle-cell disease
Spinal m uscular atrophy
Blood cel l disorder; chronic pain and infections
Severe, usually lethal p rogressive m u scle-wasting di sorder in children
Spinocerebel lar ataxia, type 1
Tay-Sachs disease
Seizures, paralysis; fatal neurological disease of early child hood
Thalassemias
Anemias
Decreased motor skills, cogn itive impairment, early death
I nvolu ntary muscle movements, reflex disorders, explosive speech
These are susceptibility tests that provide only an estimated risk for developing the disorder.
(Source: Human Genome Project, 2006, http ://www.oml.gov/scl/techresources/H uman_Genome/medicine/genetest.shtml.)
0
1 . The h u m a n gen e t i c cod e , t ra n s m i t ted a t t h e m o m e n t of
conce p t i o n , i s stored 1 11 o u r genes and i s co m posed of
specific seq u e n ces o f ____
V N C l :.l clA\SllV
2. ____ t w i n s a re ge netica l l y i d e n t i c a l a n d c o m e from
the same zygote.
:i 1 1 o � k1. u u o i,\1 :.1 at.\S U V
3 . A ____ i s t h e u n derl y i n g com b i n a t i o n o f ge netic
mate r i a l p rese n t ( b ut o u t wa r d l y i n v i s i b l e ) i n a n o rga n ­
ism, w h i l e a p h e n o t y p e i s the obse r v a b l e t r a i t .
•· To see 1 1 1 0 re re view ,1 1 1 cs t i o 1 1 s, log 0 1 1 t o lvly l >c , ,cfop 1 1 1 c 1 1 1 /_, , /1.
The l nteructio n of H ereditM
und Enviro nme nt
Like many other paren ts, Jared's mother, Leesha, and h is
father, J a m a l , t r ied to figure out which o n e of them their
new baby resembled the most. He seemed to h ave Lee­
sh a's big, wide eyes, and Jamal's genero us smile. As he
grew, Jared grew to resemb l e h i s mother and father eve n
more. H is hai r grew i n with a h a i rl i n e just like Leesh a's,
a n d h i s teet h, when they came, made his s m i le resemble
J amal 's even more. He also seemed to act l ike his par­
ents. For exa mple, he was a charming little baby, always
ready to s m i l e at people who visited the house-i ust
l ike h is fr iendly, jovial dad. He seemed to sleep like his
mom which was lucky because Jamal was an extremely
l ight sleeper who could do with as l i ttle as 4 hours a
night, ,vhereas Leesha l i ked a regu l a r 7 or 8 hours.
Were Jared's ready s m ile and regular slee p i ng habits some­
thing he j u s t l uckily i n h erited fro m h is parents? Or did Tamai
and Leesha provide a happy and stable home that encouraged
Chctp te r 3
The Start of Life: Genetics and Prenatal Development
55
M u ltifa c tori a l tra n s m is s i o n
The determ i n a t i o n of traits by a
combinat i o n of genetic and
environ mental factors in which a
genotype provides a ra nge
within which a phenotype may
be expressed
Tempera ment Patterns of
arousal and emotionality that
represent consistent and
enduring c haracteri stics i n an
i n dividual
From Research to Practice
Are " Designer Babies" i n Our Future?
Adam Nash was born to save h i s older sister Mol ly's l ife-lit­
eral ly. Molly was suffering from a rare d i sorder called Fanconi
ane m ia, w h i c h meant that her bone ma rrow was fai l i n g to
p roduce blood cel l s . This d isease can have devastat ing
effects o n young c h i l d re n , i n c l u d i n g birth defects and certa i n
c ancers. M a n y don't survive to a d u lth ood . M o l ly's best hope
for overcoming this di sease was to grow healthy bone mar­
row by rece i v i n g a transplant of i mmature blood cells from the
p l acenta of a n ewborn s i b l i n g . But not j u st any s i b l i n g wou ld
do-it had to be one with com pati ble cells that wou ld not be
rej ected by Molly 's i m m u n e system. So M olly's parents
turned to a n ew and risky tec h n i q ue that had the potential to
save M o l l y by using cel ls from her u n born broth er.
M o l ly's parents were t h e fi rst to use a genetic scree n i n g tech n i q u e
cal led preimplan ta tion genetic diagnosis (PG D) t o ensure that their
next c h i l d wou l d be free of Fancon i anemia. With PGD, a newly fer­
til ized embryo can be screened for a variety of genetic d iseases
before it is im pl anted in the mother's uterus to d evelop. Doctors fer­
tilized several of M o l l y 's m oth er's eggs with her hu sband's sperm i n
a test tube. They then examined t h e e m b ryos to ensure that they
would only i m p l ant the embryo that PG D revealed to be both genet­
ically healthy a n d a match for M o l ly. When Adam was born 9
months l ater, M o l ly got a new lease on life, too: The trans plant was
a success, and M o l l y was c u red of her d i sease .
Molly's parents were understandably focused on saving their
seriously ill daughter's l ife , but they and their doctors also opened a
controvers ial new ch apter in genetic engi neering involving t h e use
of advan ces i n reproductive medicine t h at give parents a degree of
prenatal control over the traits of their c h i l d re n . Another procedu re
that makes t h i s level of genetic control poss i b l e is germ line therapy,
i n which cells are taken from an embryo and then replaced after t h e
defective genes they contain have b e e n re paire d .
W h i l e P G D and g e r m l i n e therapy h a v e important u s e s i n t h e pre­
vention and treatment of seri ous genetic d i so rders , concerns have
been raised over whether such scientific advances can lead to the
development of "desig ner babies"-infants t h at have been genet i ­
cally man ipu lated to h ave traits t h e i r parents wish for. The q u estion
is whether these procedures can a n d s h o u l d be used not only to
correct u n d es i rable genetic d efects, but also to breed i nfants for
specific pu rposes or to " i m prove" future generat i o n s on a genetic
leve l .
T h e et hical concerns are numero u s : I s i t ri g h t to tailor babies t o
serve a specific p u rpose, however noble? Does t h i s k i n d o f genetic
control pose any dangers to the h u man gene pool? Wou ld unfair
advantages be conferred on the offspring of t h ose who are wealthy or
privileged enough to have access to these p roced ures? (Sheldon &
Wilkinson, 2004).
Designer babies are n ' t with us yet; c u rrently, scientists d o not
u n d erstand enough about the h u man genome to i d e n t i fy the genes
that control most traits, nor are they able to make g e n e t i c mod ifi ca­
tions to control how those traits w i l l be e x p ressed . Moreov er, the
term itself is a bit m i sl ead i n g . For one t h i n g , babies a re n ' t being
genetically engi neered ; PGD merely entails selec t i n g a n e m b ryo that
already has the d esired genetic makeu p . Fo r another t h i n g , i t 's a d if­
fic u l t and expensive proced u re that does not l e n d itself to casual
use. Sti l l , as Adam Nash 's case reve a l s , we are i n c h i n g c l oser to a
day when it is possible for parents to decide what gen es their c h i l ­
d ren w i l l and wi l l not have .
•
How m i g h t Adam feel w h e n h e l earn s t h a t h e was sel e c t e d to
be born i n o rder to s ave h i s si ster?
o
these welco me traits? \,Vh at causes our behavior? Nature o r
nurture? I s behavior produced b y i n h erited, genetic i n fl u ­
ences, or is it triggered by factors in the environment?
The simple answer is: Th ere is n o s i m ple a n swe r.
The Role of the Envi ronment i n
Determ i n i ng the Expression of Genes:
From Genotypes to Phenotypes
As devel opmental research accu m ulates, it i s beco m i n g
increasi ngly clear that t o view b e h av i o r a s due t o either
ge n e t i c or env i r o n m ental factors is i n ap p ro p r i ate. A given
behavi or is not ca used j ust by genetic fa c tors; n o r i s it ca used
sol ely by envi ron m e n tal forces. I n stead , as we fi rs t d i scussed
56
PART O N E
B eg i n n i n g s
How m i g h t the c i rc u m stances of Adam 's b i rt h affect t h e rela­
t i o n s h i p between h i m and M o l l y as t h ey g row u p ?
What if o u r u n derstan d i n g of t h e h u m an g e n o m e develops to
the point t h at it becomes p o s s i b l e to use P G D to control the
fut u re i n t e l l i g e n c e , attracti ve n e s s , or sexual i ty of one's c h i l ­
d ren? Where should we d raw t h e l i ne o n parents' a b i l ity t o d i c ­
t ate what traits t h e i r c h i l d re n w i l l have?
i n Chapter I , the behav i o r is t h e pro d uct o f s o m e co m b i n a ­
t i o n o f the two.
For i n stan ce, c o n s i d e r tempe ra m e n t , p a t t e r n s of a rousal
a nd emo t i o n a l i t y t h a t rep rese n t consi s te n t and e n d u ri n g
cha racte ristics i n a n i n d i v i d u a l . S u p pose w e fo u n d-as
i n c reas i n g evidence sugges t s is t h e c ase-t h a t a s m a l l per ­
centage of ch i l d re n are b o rn w i t h te m pe ra m e n ts t h a t p ro d uce
a n u n u sual degree of p hys i o l o g i c a l rea c t i v i t y. H a v i n g a ten­
dency to s h r i n k fro m a n yth i n g u n u s u a l , s u c h i n fa n t s r eact to
n ovel sti m u l i w i t h a rapid i n crease i n h e a r tbea t and u n usual
e x c i t a b i l i ty o f t h e l i m b i c s y s t e m o f the b ra i n . Such h e i g h t ­
e n ed reac t i v i t y to s t i m u l i a t t h e s t a r t o f l i fe, w h i c h see m s to
be l i n ked to i n h e r i ted facto rs , i s a l so l i ke l y to c a u se c h i l d re n ,
by t h e t i me t h e y a re 4 o r 5 y e a r s o l d , tu be co n s i d e rt?d shy by
t h e i r pa rents a n d tea c h e r s . B u t n o t a l w a y s : S o m e o f t h em
beh ave i nd i s t i n g u i s h a b l y fr o m t h e i r peers a t t h e same age
( Kaga n & S n i d m a n , 1 99 l ; McCrae et al., 2000 ) .
Wh a l m a kes t h e d i fferen c ? T h e an s wer seems t o b e t he
en v i ron m e n t in which the ch il dren a re raised. Children, whose
p a ren ts encourage them to be ou tgo i ng by arran gi n g new
o p p o rt u n i t ies for them, may ove rcom e thei r s hyness . I n co n ­
t rast , ch i l d ren rai sed i n a st ressful env i ro n m ent m arked by
m a r i tal d i sco rd or a prolonged i l l ness may be more l i kely t o
ret a i n the i r shynes later in l i fe ( Ka ga n, A rcus, & S n idman,
1 993; R. Joseph, 1 999; Propper & M oo re, 2006 ) . Jared, des c r i b ed
e·irlier, may have been b o rn with a n easy temperame n t, w h i c h
was rea d i l y rei n forced by his cari n g p arents.
I nte raction of Factors Such fi n d i n gs i llustrate th a t many
t ra i t s refl ect rn u l t i fa ctoriaJ t ra n s , · ion, mea n i n g that they
are determ i 1iecl by .�inat i o n o f b o t h oenetic a n d e nv 1 ro n ­
rfl e rit,il facto rs:- m mu lf i fact orial t ra n sm issi o n , a genotype pro ­
v ides a part icu ,11.--r,1 11ge within \v h ich a p h enot ype ma)' a ch ie ve
exp ressio n . Fo r instance, peopl e wi t h a genotype t h at p e r m i t s
t h e m to ga i n weigh t eas i l y may n eve r be sli m , no mat ter h ow
much t hey diet. They m ay be relatively s l i m , given t h eir ge n et i c
heri tage, b u t they may never be ab l e to get beyo nd a certai n d e ­
gree o f t h i n ness ( Fa i t h , J o h nson , & Allison, 1 997 ) . In many
cases, t h e n , it is the environment that determ i nes the way i n
which a particular ge n o t ype will be expressed as a p he notyp e
( Wachs, 1 99 2 , 1 993, 1 9 96; P l o m i n , 1 9946 ) .
On t h e o t h e r hand, certain genotypes a re relatively unaf­
fected by environmental factors. In such cases, development
fo l l ows a preorda i n ed pattern, relat ively i n depend e n t o f the
spec i fi c env i ro n m en t i n which a perso n is raised. For instan ce,
resea rch on preg n a n t wome n who were severely malnour­
ished d u r i n g fa m i n es caused by World War I I found that the ir
N a t u re
chil d ren were, o n average, unaffected physically or intellectu­
ally as adults (Z. Stei.n et al., 1 975 ). Similarly, no matter h ow
m u ch health food people eat, they are not going to grow
b eyond certain genetically in1posed limitations in height. Lit­
tle Jared's hairline will p robably be affected very little by any
ac t ions on the part of his parents. Ultimately, of course, it is
the u n ique i nteraction of inherited and environ mental factors
that determ ines people's pa tterns of development.
The more a p p ro p r i a te question, then, is how m uch of
the behavior is caused by genetic factors, and how much by
env i ro n m ental factors? ( S ee, fo r exa mple the range of possi­
b i l it ies fo r the determi n a n ts o f in telligence, illust rated i n
Figure 3 - 8 . ) A t one extreme is t h e idea t h a t oppo rtun ities i n
the e nv i ro n me n t are solely responsible fo r intel ligence; o n
the other, that i n telligence is p u r e l y genetic-you either have
it or you don't. The usefulness of such extremes seems to
p o i n t us toward the middle gro u nd-that i ntelligence is
the r e sult o f s o m e comb i n ation of natural mental ability a n d
environmental opportunity.
Studyi ng Development: How M uch
Is Nature? How M uch Is Nurtu re?
Developmental researchers use several st rategies to try to resolve
the question of the degree to which traits, character istics, and
behavior are produced by genetic or environmental factors.
Their studies i nvolve both nonhuman species and humans.
N o n h u m an An i mal Studies: Contro l l ing Both G eneti cs
and Enviro n m ent It is relatively simple to deve lop breeds
of animals that are genetically sim ilar to one another in terms
of spec i fi c traits. The people who raise B utterball turkeys for
----
I n t e l l i g e nce i s prov ided
entirely by genetic fac­
tors; environment pl ays
no ro le. Even a h i ghly
enriched environment
� a n d exce l l ent e d u cation
:fl m a ke no d i fference.
N u rt u re
Although larg e l y
i n h e rited, i ntelligence
is affected by an
extremely en riched or
depr ived enviro n m ent.
.!!:!
.c
'iii
V,
0
CL
I ntel l i gence is affected
both by a person's
genetic e n d owment a n d
envi ronment. A person
geneti cally predisposed
to low intelligence may
perform better if raised
in an enriched e n v i ro n ­
ment o r w o r s e i n a
dep rived environm ent .
S i m i l arly, a person
genetica ll y p red is posed
to h i g her intellig ence
may perform worse i n
a deprived e n v i r o n m e nt
or better i n a n en riched
enviro n m ent.
Altho u g h inte l l igence
is l a rg e l y a result of
enviro n m e nt, g e n etic
a b normalities may
prod uce menta l
retardati o n .
Inte llig ence depends
entirely o n the environ­
m e nt. G e n etics
p l ays no role in
d eterm i n i n g
i ntel l ectual success.
F I G U R E 3 · 8 Poss i b l e Causes of I nte l l igence
Intelligence may be explai ned by a range of differing poss ible sources, spanning the nature-nurture con t i n u u m . Which of tl1ese
explanations do you find most convi ncing, g i ven the evidence discussed i n the chapter?
Ch CI.p te r 3
The Start of Life: Genetics and Prenatal Deve l o pment
57
Thanksgiving do it all the time, producing turkeys that grow
especially rapidly so that they can be b rought to market inex­
pensively. Simil arly, strains of laboratory animals can be bred
to share similar genetic backgrounds.
By observing animals with similar genetic backg rounds
in different environmen ts, scientists can determin e, with
reaso nable precision, the effects of specific kinds of environ­
mental stim ulation. For example, animals can be raised in
unusually stimulating environmen ts, with lots of items to
climb over or th rough , o r they can be raised in relatively bar­
ren envi ronments, to determ ine the results of living i n such
different settings. Co nversely, researchers can exam i ne
groups of animals that have been bred t o h ave significan tly
different gen etic backgro unds on particular traits. Then, by
exposing such animals to identical environments, they can
determin e the role that genetic backgro u n d plays.
Of co urse, the drawback to using n o n humans as
research subjects is th at we can't be su re how well the find­
ings we obtain can be generalized to people. Still, the oppo r­
tunities that an imal research offers are substantial.
Contrasting Related ness and Behavior: Ado ption,
Twi n , and Fam i ly Stu dies Obviously, researchers can't
control either the genetic b ackgro unds or the envi ronments of
humans in the way they can with nonhumans. However, na­
ture conveniently has provided the potential to carry o u t vari­
ous kinds of "natural experi ments"-i n the form of twins.
RecaU that identical , monozygotic twins are also identical
genetically. Beca use their i nherited backgro u nds a re precisely
the same, any vari ations in their behavior m ust be due entirely
to envi ron mental facto r
58
PA RT O N E
Beg i n n i n gs
It would be rat her s i m p l e for resea rc h e rs lo mnke u ·e of
identical twin to d raw u neq u ivocal co n c l u s i o n s about the
roles of nature and n u r t u re. Fo r i n sta nce, by scp:1ra t i ng iden­
tical twins at b i r t h and placing 1 h c m in t o l a l l y d i ffcrc n l envi­
ron ments, researchers could assess t h e i m pact of
enviro n ment u n a m biguously. Of cou rse. e t h ical considera­
tions make t h i s i m possible. I- 1 0\-v cvcr, w h a t rL'searcher
can-and do-st udy, are cases in wh ich idL'n l i cal ! \\'i n s have
been put up for ado ption ·1 1 b i r t h a n d a rc rai. eel i n substan­
tiall y different env i ronmen ts. Such i nsta nces a l l ow us to
draw fairly confident concl usions abo u t t hL' rela t ive contri­
butions of genet ics and env i ron men t ( Ba i l ey ct al., 2000;
Richardson & Norgate, 2007 ) .
The data from such stud ies of ident ical twins raise d in
d i fferent environments are not always w i t hout bias. Adopt ion
agencies typically take the characteris t i cs ( a n d wishes ) of birth
mothers i n to acco u n t when they place bab i es in ado ptive
hom es. For instance, children tend to be placed w i t h fa milies
of the sam e race and rel igion. Conseq uent ly, even when
mon ozygotic tw i n s are placed i n d i ffe re n t ado ptive homes,
there are often s i m i l arities between the two home env iron­
men ts. As a resul t, resea rchers ca n't always be cen ai n t hat dif­
ferences in behavior a re due to d i ffe rences i n t he env i ro n m ent.
St udies of nonidentical, d i zygotic tw i ns a l s o presen t
op portu nities to learn about the rela t i ve co n t rib ut i on s of
natur e and n u r ture. Recall that d i zygotic twi ns a re gene t ical ly
no more sim ilar t han sibl i ngs in a fa m i l y born a t diffe rent
tn:n es . By co mpa ring behavior w i t h i n pai rs of d i zygot ic tw i ns
�vith � hat of pa irs of mono zygotic twins ( who a re gene tic al � y
ident ICal ) rese archer s can determ i n e whether mo no zygo tJC
twins are more si m i l ar on a particu lar trait, on average, th a n
?i zygot i c twins. If so, t hey can as s ume tha t genetics p l a ys a n
imp ortant role in determining the exp ression o f t hat t rait.
Still a nother approach is to stud y people who a re tota lly
unrel ated to one another and who therefo re have dissi mi lar
geneti c backg ro unds, but who s h a re an enviro n me ntal back­
ground.
For instan ce, a family that adopts, a t the same
_
time � two very young unrelated ch ildren p ro babl y will
prov ide them with quite similar env i ron ments throu gho ut
their childho od. In this case, similari ties in t h e ch ildren's cha r­
acteristi cs and behavi or can be attributed with some confi ­
dence to env ironm ental influen ces ( N . L. Segal , 1 993 , 2000) .
_
Fmally, develo pmental researcher s have exa m ined groups
? f people _ i n light of their d e g ree o f ge netic simila r i ty. Fo r
mstance, if we fi n d a h igh association on a pa rticu lar trait
betw een biological parents and thei r children , but a weake r
associatio n between adoptive pare nts and their chi ldren , we
have evide � ce for the impo rtance o f genetics i n determ i n i ng
the express10n of that trait. On the other hand, i f there is a
stronge r association on a trait betwee n adoptive parents and
their ch i l d ren than between biological parents a n d thei r ch i l ­
dren, we have evidence for t h e importance o f the enviro n ment
i n determ i n i ng that trait. I f a particular t rait tends to occur at
similar levels amon g genetical l y similar individua ls, b ut occurs
at d i fferent levels among ge net ically more d i stant i ndivid uals,
signs poi n t to the fac t that genetics plays an important role in
the development o f that I ra it ( Rowe, l 994 ) .
Developmental resea rchers have used these approaches
and others 10 st udy the relat ive i m pact of genetic and envi ron­
mental factors. \Vhat have they found?
Before t u rn ing to speci fic fi n d i ngs, here's the general con­
clusion resu l t i ng from decades o f research: Virtually all tra its,
characterist ics, and behav iors a re the joint res u l t of the com­
b i nation and i n teract ion of natur e and nurt ure. Genetic and
environ m e n t a l factors wo rk in tandem, each affect ing and
being a ffected b y the oth er, c reating the u n ique individual
that each of us i s and will become (G. E. Robi nson, 2004;
Waterl and & J i rtle, 2004 ) .
Over a 3-month period, t h e twins gained
almost identical amounts of weight. Moreover,
different pairs of twins varied substantially i n
h o w much weight they gained, with some pairs
ga ining almost three ti mes as much weight as
other pairs ( Bouchard et al., 1 990 ) .
Other, less obvious physi cal characteristics
also show strong genetic i nfluences. For
instance, blood p ressure, resp iration rates, and
eve n the age at which l i fe ends a re m o re similar
in closely related i n d ividuals than in those wh o
are less genetically alike ( Sorensen et a l . , 1 988;
Price & Gottesman, 1 99 1 ) .
Phys ical Traits: Fam i ly Resemb lances
I nte l l i gence: M o re Research ,
M o re Controversy
'vV h e n pa t i e n t s e n tered the examining ro o m of Dr. Cyril
Ma rc us, t hey didn't rea l i ze t h a t sometimes t h ey were actually
being t reated by h i s ident ical tw i n b rother, D r. Stewa rt Mar­
cus. So si m i l a r i n a ppea rance and manner were the twins
that even l o n g - t i me patients were fooled by this admittedly
u nethical behavior, wh ich occurred in a b izarre case made
famous in the fi l m Dead Ri1 1gers.
Monozygo t i c twins a re merely the most ext reme
exam ple of the fact that the more geneticall y similar two peo­
ple are, the more l i kely they are to share physical character is­
tics. Tal l parents tend to h ave tall children, and short ones
tend to have short c h i ldren. O6es1ty, which is defined as·oemg
mo re t h a n 20% above the average weight for a given height,
also has a st ro n oo ooenetic compo nent. For example, in one
. t h at c onta1ne
. d
study, p a i rs of identical twins were put on d 1ets
to
not
exercise.
ordered
day-and
an ex tra l ,000 calories a
No other issue i nvolving the relative influence
o f h eredity and environment has generated
more research than the topic of intelligence. Some traits-like curly hair­
Why? The m a i n reason is that intelligence, gen- have a clear genetic
erally measured in terms of an IQ score, is a component.
central h u man characteristic that differentiates
h u m ans from other species. I n addition, intelligence is strongly related to success in scholastic endeavo rs
and, so mewhat less strongly, to other types of achievement.
Genetics plays a ign i fi c a n t role in intelligence. In
studies o f both general intelligence and of specific sub­
componen ts of i n telligence ( such a s spatial skills, verbal
skills, and memory ) , t h e closer the genetic link between
two individu als, the greate r the correspo ndence of their
overa l l IQ scores. ( See Figure 3 - 9 )
F I G U R E 3 - 9 Genetics and I Q
The closer the genetic link between two individuals, the greater the correspondence between their IQ scores. Why do you think there is a
sex d ifference in the fraternal twins' figures? M ight there be other sex differences in other sets of twi ns or siblings, not shown on this chart?
(Source: Bouchard & McGue. 1 98 1 )
C h ct p te r 3
The Start of Life: Genetics and Prenatal Development
59
Not only is genetics an impo rtant i nfluence
o n i ntelligence but also the impact increases
with age. For instance, as fraternal ( i .e., d izygotic) twins move from i n c1 ancy to adolesSome people have used
cence, their I Q scores become less simila r. I n
the proven genetic basis
contrast, the IQ scores of iden tical
of intelligence to argue
( monozygotic) tv1ins become increasi ngly
against strenuous educational
similar over the cou rse of time. These
efforts on behalf of ind ividuals
opposite patterns sugges t the i:itensifying
with below-average IQs . Does
_
_
.
' h1s v1ewpo1nt make sense / mfluence of mhented factors with mcreasbased on what you have
ing age ( Brody, 1 993; McGue et a l . , 1 9 93 ) .
learned about heredity
Al though it is clear that h e redity p lays
and environment?
an important role in inte 11 igence, i nvestiga­
Why or why
tors are much more divided on t h e question o f
not?
the degree t o which i t i s i n h erited. Perhaps the
most extreme view is held by psychologist Arthur
Jensen ( 2003 ) , who argued that a s m uch as 80% o f i n tel l i ­
gence i s a result of heredity. Others have suggested more
modest figures, ranging from 50% to 70%. I t is critical to
keep in mind that such figures a re averages across large
gro ups of people, and any particular i n dividual's degree of
inheritance cannot be predicted from these averages ( e.g.,
Herrnstein & Murray, 1 994; Devl i n , Dan iels, & Roeder, 1 99 7 ) .
From
an educator's
perspective:
I
Social potency
61 % 1
A person h ig h in this trait is masterful, a forceful leader who li kes to be the center of attenti o n .
I
Traditionalism
60% 1
Foll ows rules and authority, endorses high moral standards and strict disci p l i n e .
I
I
I
I
I
s s o/o 1
Stress react.ion
55%
Feels vul nerable and sensitive and is g iven to worries a n d is easily u pset.
It is important t o keep i n m i n d that a l t hough hered ity
clearly plays an i mportant role i n i n tell igence, envi ro n men tal
factors such as exposure to books, good educa t i onal experi­
ences, and intel l i ge n t peers a re p ro fo u n d l y i n fl u e n t i a l . Even
t hose like Jensen who make the m ost ex t reme est i m a tes of the
role of ge netics sti l l al low for enviro n m e n ta l factors to play a sig­
nificant role. In fact, in terms of public p o l i cy, env i ro n m ental
i n fl uences are the focus of efforts geared towa rd maxi m i zing
people's intellect ual su ccess. As developmental psychologist
Sandra Sca rr suggests, we should be asking what can be clone to
maxim ize the i n tellectual devel opment of each i nd ividual
( Scarr & Carter-Sal tzman, 1 98 2 ; Sto rfer, 1 9 90; Bouchard , 1 9 97 ) .
Genetic and Environm ental I nfl uences
on Personal ity: Born to Be Outgoing?
D o w e i n herit o u r personal i ty?
At least in part. There's i n c reasing resea rch evidence sug­
gesti ng that some of our most basic perso n a l ity traits have
ge netic roots. For example, two of the key " B ig Five" persona li ty
traits, neuroticism and extroversion, have been l i n ked to genetic
factors. Neurot icism, as used by personality researchers, is the
degree of emotional stabil ity an i n d ividual cha racterist ically
displays. Extroversion is the degree to which a person seeks to be
with oth ers, to behave i n an outgoing manner, and gen erally to
be sociable. For instance, Jared, the baby described ea rl ier i n this
chapter, may have i n h erited a tendency to be o u t go i n g from his
Absorption
55%
Has a vivid imagination read i l y captured by rich experience; re linqu ishes sense of real ity.
Alienation
Feels mi streated and used, that "the world is out to get m e . "
I
Well-being
Has a cheerful disposition, feels confident and opti m i stic.
s4% 1
I
ext roverted fa t her, J a m al ( Plomin & Caspi, 1 998; Benja min,
Ebstei n, & Bel m a kc r, 2002 ; Zuckerman, 2003 ) .
How do w e know w h i c h personality traits reflect genetics?
Some evidence comes from d i rect exam i n at ion of genes them­
selves. For i n sta nce, it appea r · t h at a spec i fic gene is very influ­
ential in determ i n ing risk-taking behavior. T h i s novelty- seeking
gene a ffects the production of the b ra i n chemical dopamine,
making some people m o re prone than others to seek out novel
situations a n d to take risks ( Ebstein et a l . , I 996; Gi llespie et al,
2003; Serre t t i ct a l . , 2007 ) .
Other evidence for t h e ro le o f ge netics i n t h e determina­
tion of personality t ra its comes from studies of twins. For
instance, in one major · tud y, researchers looked at the personal­
i ty tra i ts of h u nd reds of pairs of twi ns. Because a good number
of the twi n s were ge net ically identical but had been raised apart,
it wa possible to d etermine with some confidence t he influence
of genet ic factors ( Tellcgen ct al., 1 9 8 8 ) . The researchers found
that certa i n t ra i ts reflected the contribution of genetics consid­
erably more than other . As you can see i n Figure 3- 1 0, social
potency ( the tendency to be a masterful, forceful leader who
enjoys being the ce nter of atte n t i o n ) and tradit ionalism (strict
endorsement of ru les and authority) are strongly associated
with ge netic factors ( H arris, Vernon, & Ja ng, 2007 ) .
Even less b a s i c perso nality tra i ts are linked t o genetics.
For exa m p le, p o l i t ical a t t itudes, rel igious in terests and values,
and eve n a t t i t udes towa rd human sexuality have ge netic com­
ponents ( Eley, 2003; Bouchard, 2004; Koenig et al., 2005 ) .
Clearl y, genetic factors play a role i n determ i n i ng per­
sonal i t y. At t h e same ti me, the env i ro n m ent i n wh ich a ch ild
is raised also a ffects persona l ity development. For example,
some pare n t s e n c o u rage high activity levels, seeing activity as
a m a n i festation of i n dependence and intel l igence. Ot h er par­
ents m ay encourage lower levels of activity on the part o f
t h e i r ch i l d ren, feel i n g t h a t more passive chil d ren will get
along better in soci ety. Pa rt of these paren tal attit udes are cul­
turally determ i ned ; p a rents i n the United S t ates may encour­
age h i gher activity levels, whereas parents i n Asian cult ures
may enco urage grea ter passivi ty. In both cases, children's per­
sonalities will be sh aped in part by their p arents' attitudes.
Because both genetic and environmental factors have con­
sequences for a ch ild's personality, personality development is a
perfect example of a central fact of child development: the
interplay bet:\veen nature and nurture. Fu rthermore, the way in
which nature and nurture interact can be reflected not only in
the behavior of individuals, but also in the very foundations of a
culture, as we see next.
D I V E R S I TY
Cultural Differences i n P hysical Aro usal :
M i ght a C u ltu re 's P h i l osophical Outlook
Be Determ i ned by Genetics?
The Buddhist philosophy, an inherent part of many As ian cul­
tures, em phasizes harmony and peacefu lness. I n contrast,
some traditional Western philosophies, such as those of Martin
Luther and John Calvi n , accent uate the i m portance of control­
ling the anxiety, fear, and g u i lt that they assume to be basic
parts of the human conditi on.
Could such philosophical approaches reflect, in part, genetic
factors? That is the controversial suggestion made by devel opmen­
tal psycholog ist Jerome Kagan and his colleagues. They speculate
that the underlying tem perament of a given society, determined
geneti cally, may predispose people in that society toward a particu­
lar philosophy (Kagan , Arcus, & Snidman, 1 993; Kagan, 2003).
Kagan bases h is adm ittedly speculative suggestion on well­
confirmed findings that show clear differences i n tem perament
between Caucasian and Asian c h i l d ren. For i n stance, one st udy
that com pared 4-month-old i nfants i n China, Irel and , and the
United States found several relevant d ifferences. I n comparison
to the Caucasian American babies and the I rish babies , the Chi­
nese babies had sign ificantly lower motor activity, irritability,
and vocalization (see Table 3-4) .
Kag an suggests that the Ch i nese, who enter the world tem­
peramentally calmer, may find Buddhist philosoph ical notions of
Harm avoidance
so% 1
Shuns the excitement of risk a n d danger. prefers the safe route even if it is ted ious .
I
Agg ression
Is physically agg ressive- and vind ictive, has ta ste for violence and is '"out to get the world . "
M ean Behavioral Scores for Caucasian Am erican , I ri s h ,
a n d C h i nese 4-M onth-Old I nfants
I
Ach ievement
46 % \
Wo rks hard, strives for m astery, and puts work and accomplishment ahead of other things.
I Control
Be havior
I
43 %
Is ca utious and plodding, rational and sensible, l i kes carefu l ly planned events .
I
Social closeness
33% 1
P refe rs emoti o n a l intim acy and close ties, turns to oth ers for comfort and help.
F I G U R E 3 - 1 0 I n herit i n g Traits
These traits are among the perso nal ity factors that are related most closely to
genetic factors . The higher the percentage, the greater the degree to which the
trait reflects the influence of hered ity. Do these figures mean that "leaders are
bor n . not made"? Why or why not?
(Source: Adapted from Tellegen et al. . 1 988)
60
PA RT O N E
Beg i n n i n g s
American
Irish
M otor activity score
48.6
36.7
1 1 .2
Crying (in seconds)
7.0
2.9
1 .1
Fretting (% trials)
100
6.0
1 .9
Vocal izing (% trials)
31 .4
31 . 1
8.1
4.1
2.6
3.6
S m i l i ng {% trials)
"The good news is that you will have a healthy baby girl.
The bad news is that she is a congenital liar. "
Chinese
(Source: Kagan , Arcus. & Snid ma11 , 1 993)
Cl1.C?i.µ te r 3
The Start of Life: Genetics and Prenatal Development
61
serenity more in tune with their natural inclinations. In contrast ,
Westerners, who are emotionally more volatile and tense, and
who report higher levels of guilt, are more likely to be attracted to
philosophies that articulate the necessity of controlling the
unpleasant feelings that they are more apt to encou nter in their
everyday experience (Kagan et al., 1 994; Kagan , 2003) .
It is important to note that this does not mean that one philo­
sophical approach is necessarily better or worse than the other.
Nor does it mean that either of the temperaments from which the
philosophies are thought to spring is superior or inferior to the
other. Similarly, we must keep in mind that any single ind ivid ual
within a culture can be more or less tem peramentally volatile and
that the range of tem peraments found even with in a particular
culture is vast. Final ly, as we noted in our initial discussion of tem­
perament, environmental conditions can have a sign ificant effect
on the portion of a person 's temperament that is not genetical ly
determi ned. But what Kagan and his colleag ues' speculation
does attempt to address is the back and forth between culture
and temperament. As religion may help mold tem perament, so
may temperament make certain religious ideals more attractive.
The notion that the very basis of culture-its philosophical tra­
ditions-may be affected by genetic factors is intriguing. More
research is necessary to determine just how the unique interaction
of heredity and environment within a given culture may produce a
framework for viewing and understanding the world.
62
Lifetime R i s k of Developing Schizophrenia (percent)
General population
Spouses of patients
Fi rst cousins
Uncles/au nts
1%
2%
2%
2%
Nephews/nieces
4%
Grandchildren
Half-siblings
Children
Sibli ngs
Siblings with 1 parent
with schizophrenia
Dizygotic twins
S%
13%
9%
Parents
6%
Monozygotic twi ns
48%
6%
17%
17%
0
10
20
30
Degree of Risk
40
so
F I G U R E 3 - 1 1 The Genetics of Schizophre n i a
Psychologi cal D i so rders: The Role
of Genetics and Enviro n ment
The psychological disorder of schizophrenia has clear genetic
components. The closer the genetic links between someone with
schizophrenia and another family member, the more likely it is that
the other person will also develop sch izophrenia.
(So urce: Gottesman. 1 99 1 )
Lori Schiller bega n t o hear voices when s h e was a
teenager in summer camp. Without warning, the voices
screamed "You m ust d i e ! Di e! Die!" She ran fro m her
b unk i n to the da rkness, where she thought she could
get away. Camp counselors found her screaming as
she j u mped wildly on a trampoline, "I thought I was
possessed ," she s a i d l ater. ( Bennett, 1 992 )
I n a sen se, she was possessed: possessed with schizophren ia,
one of the severest types of psych ological disorder. Normal
and happy through ch ildhood, Sch i l l er's wo rld took a tumble
d uring adolescence as she increasingly lost her hold on real ­
i ty. F o r the next 2 decades, s h e wo uld b e i n and o u t of i nsti t u ­
tions, struggl i n g t o ward o ff the ravages of t h e diso rder.
What was the cause of Sch iller's mental disorder? Increas­
ing evidence suggests that schizophrenia is b ro ught about by
genetic factors. The disorder runs in families, with some fami­
lies showing an u nusually h igh incidence. Moreover, the closer
the genetic links between someo ne with sch izoph ren ia a n d
another fa m i ly member, t h e more l ikely i t is that t h e other per­
son will also develop schizophrenia. For i nstance, a monozy­
gotic twi n has close to a 50% risk of developing sch izo p h renia
when the oth er twin develops the disorder ( see Fi gure 3 - J l ) .
O n the other hand, a n iece o r nephew o f a person w i th schizo ­
ph ren ia has less than a 5 % chance o f develo ping the disorder
( Prescott & Gottesman, l 993; Han son & Gottesman, 2005 ) .
However, the e d a t a a l so i l l ust rate t h a t ge n e t i cs a l o n e
doe n o t i n fl uence t h e develo pment o f t h e d i so rd e r. I f gen et­
ics were the sol e c a u se, the risk for a n i d e n t ical twi n wo u l d
b e 1 00%. Consequent ly, other factors a c co u n t fo r t h e d i so r­
der, ra n g i n g from struc t u ral abnorma l i t i es i n t h e b ra i n to a
b i ochemical i mbalance ( e.g., Lyo ns, 13a r, & Kremen , 2002;
H i etala, Can n o n , & va n Erp, 2003 ) .
I t also seems that even i f i ndivid uals h a rbor a genetic pre­
disposition towa rd schizophren ia, they are not desti ned to
develop the disorder. Instead , they may i n herit an u n usual sen ­
sit ivity t o stress in t h e envi ronment. I f stress i s low, schizophre­
nia will not occur. But if stress is sufficiently strong, it w i l l lead
to schizoph renia. On the other hand, for someone with a s t rong
genetic predisposi tion toward the diso rder, even relatively weak
environ mental stressors may lead to sch izophren ia ( Paris, 1 999;
Norman & Malla, 200 l ).
Several other psychological disorders have been show n to
be related, at least in part, to genetic factors. For i nsta nce, major
depression, alcoholism, autism, and attention -defici t/hyperac­
tivity disorder have significa n t i n h erited components ( Prescott
et al., 2005; Dick, Rose, & Kaprio, 2006; Monastra, 2008 ) .
The example o f sch izophrenia and other genetically related
p ychological disorders also illustrates a fu ndamental princi ple
regarding the relationship between heredity and environm ent,
one that underl ies much of our previous discussion . Spec i fi ­
call y, t h e role of genetics i s often t o prod uce tendency towa rd a
fu ture co u rse of develo pmen t . \,Vh en and whether a certain
behavioral characteristic will actually be displ ayed depends on
the nature of the enviro n m ent. Th us, although a pred isposit ion
PA RT O N E
B e g i n n i ng s
Developmental
psychologist Sandra Scarr
arg ues that children's
genetic characteristics
actively influence and
shape their
environ ment.
for schizophrenia may be present at bi rth, typically people do
not show the disorder until adolescence-if at all.
S i m i l a rly, certain other kinds o f traits are more likely to
be displayed as the i n fl uence o f parents and other socializing
fac t o rs decl i n es. For example, ado pted children may, early in
their lives, display t r a i ts that are relatively similar to their
adopt ive parents' t ra i ts, given the overwh elm ing influence of
the envi ro n m e n t o n you ng children. As they get older and
their p a ren ts' d ay- to -day i n fl uence declines, ge netically
influe nced t r a its m ay begin to manifest themselves as unseen
gen etic fac tors begin to play a grea ter role ( Caspi & Moffitt,
1 993; Arsen a u l t et a l . , 2003; Pou lton & Caspi , 2005 ) .
Can Genes I nfluen ce the Environment ?
Accord ing t o develop m e n tal psycholog is � Sandra _ Scarr
( I 993, 1 998 ) , the genetic endo wment provided to children
by their p a rents not o nl y d eterm i n es their genetic character ­
ist ics, but a l so act ively i n fluences their em'. i ro n � en t. S � arr
suggests th ree ways a child's genetic prechspo s1t1011 might
i n fluence h i s or her env i ronment.
Children tend to act ively focus on those aspects of their
environment that are most connected with their genetically
determined abilities. For example, an active, more aggressive
child will grav i tate toward sports, wh ile a m_ore reserve � ch ild
_
will be more en gaged by academics or solitary pursuits like
_
computer games or drawing. Children also pay less attentw n
to those aspects of the environment that are less �ompat1ble
with their genetic endowment. For instance, two girls 1:1 ay be
reading the same school bulletin board. One may notice the
sign advertising tryo uts for Little League baseba � , wher� as her
less coordinated but more musically endowed fnend might be
more apt to spot the notice recruiting students for an after­
school chorus. In each case, the child is attending to those
aspects of the environment in which her genetically determined
abilities can flourish.
I n some cases, the gene-environment influence is more pas­
sive and less direct. For example, a particularly sports-orit::nted
parent, who has genes that promote good physical coordination,
may provide many opportunities for a child to play sports.
Finally, the genetically driven temperament of a child
may evoke certain enviro nmen tal infl uences. For instance, an
infant's demanding behavior may cause parents to be more
attentive to the in fant's needs than they would be if the in fant
were less demanding. Or, for instance, a child who is geneti­
cally inclined to be well co ordinated may play ba!I with any­
thing in the house so often that her parents notice. They may
then decide that she should have some sports equipment.
ln sum, determining whether behavior is primarily attrib­
utable to nature or nurture is a bit like shooting at a moving tar­
get. Not only are behaviors and traits a joint outcome of genetic
and enviromnental factors, but also the relative influence of
genes and environment for specific characteristics shifts over
the comse of people's lives. Altho ugh the pool of genes we
inherit at birtl1 sets the stage for om future development, the
constantly shifting scenery and tl1e other characters in our lives
determine j ust how our development eventually plays out. The
environment both influences our experiences and is molded by
the choices we are temperamentally i nclined to make.
REVI EW
.J
1 . Mapping the gene sequence has provided support for the
field of ____ genetics, which studies the effects of
heredity on psychological characteristics.
j\'..lO!Al'.ljdq :JdMSUV
2. ____ counsel ing focuses on helping people deal with
issues related to inherited disorders.
syn ­
3. Examples of i n herited disorders are
drome, once referred to as mongolism; fragile X syn drome; sickle-cell anem ia; and Tay-Sachs disease.
Ui\"\OQ :Jd,1,\SUV
4. Fo r women wh o are already pregnant, the health of the
unborn child can be assessed using ---� chorionic
vi!Jus sampli ng, or ultrasound sonography.
S!Sd)lld) O) lllll\'. :J;J,1,\SUV
* To see 111 o re re view q11esrions, log 0 1 1 to J\llyDe velop111e11tLnb.
Prenatal G rowth and Chanf}e
Robert accompanied Lisa to her fi rst appointment with
the midwife. The midwife checked the results of tests
done to confirm the couple's own positive home pregnan­
cy test. "Yep, you're going to have a baby," she confirn1ed,
speaking to Lisa. "You'll need to set up mon thly visits fo r
the next 6 mon ths, then more frequently as your due date
approaches. Yo u can get this prescription ti.w prenatal vi ­
tami ns filled at any pharmaq and here a re some guide­
li nes about diet and exercise. Yo u don't smoke, do vo u 7
1,
Ch.apter 3
The Start of Life : Genetics and Prenatal Development
63
Ferti lizati on The process by
which a sperm and an ovum­
the male and female gametes,
respectively-join to form a
single new cell
Germ ina l stage The fi rst , and
shortest, stage of the prenatal
period , which takes place during
the first 2 weeks fol lowing
conception
P l acenta A conduit between
the mother and fetus, providing
nourishment and oxygen via the
umbilical cord
That's good." Then she turned to Robert. " How about
you? Do yo u smoke?" After giving lots of instructions and
advice, she left the couple feeling slightly dazed, but ready
to do whatever they could to have a healthy baby.
Fro m the moment of conception, d evelopment proceeds
relentlessly. As we've seen , many aspects are guided by the
complex set of genetic guidel i n es inherited from the parents.
Of co urse, prenatal growth, like all development, i s also
influenced from the start by environmental facto rs. As we
will see later, both parents, like Lisa and Robert, can take part
in providing a good prenatal environment.
Fert i l ization : Th e Moment
of Concepti o n
When mos t o f us t hink about the facts of l i fe , we tend to
focus on the even ts that cause a male's sperm cells to beg i n
their jou rney toward a female's ovu m . Ye t the a c t o f sex t h a t
b rin gs about t h e potential fo r conception i s b o t h the co nse­
qu ence and the start of a long string of events that p recede
and follow fertilization , or conception: t h e j o i n i n g o f
sperm and ovum t o create t h e single-celled zygo te fro m
which each of us bega n our l ives.
Both the male's sperm and the femal e's ovum co m e w i t h
a histo ry of their own. Fem ales are b o r n w i t h aro u n d
400,000 ova located in the two ovaries ( see Figu re 3 - 1 2 fo r
the basic anatomy o f the female reproductive o rga n s ) . How­
ever, the ova do not mature until the female reaches puberty.
Fro m that point until sh e reaches menopa use, the fe male
will ovulate about every 28 days. During ovu l a t i o n , a n egg is
released from one of the ovaries and pushed by m i n ute hair
cells thro ugh the fallopian tube toward the u teru s. I f the
Fal lopian tube
Zygote
isk
Tro p h o b l ast
Soft uteri n e
lining
Uterus
Ovary
Vag i n a -----
�
U R E 'J
.t:. Anatomy of the Female Reproductive Organs
The basic anatomy of the female reproductive organs is illustrated in this cutaway view.
& Persa u d . 2003)
{Source: Moore
64
PA R T O N E
Beg i n n i n g s
E m b ry o n i c sta g e The period
from 2 to 8 weeks fol lowing
fertil ization during which
sig nificant growth occurs in the
major organs and body systems
ovum meets a sperm in t h e fa l l o p i a n t u be, fe rt i l i za t i o n takes
p l ace ( A i tken, 1 995 ) .
Sperm , which l ook a l i ttle l i ke m i c rosco p i c tad poles,
h ave a shorter l i fe span. They are created by the test i c l e s a t a
rapid rate: An a d u l t m a l e typically p rod uces seve ral h u nd red
m i l l io n sperm a day. Co nsequent l y, t h e sperm ej a cu lated
d u r i ng sexual i n tercou rse a re of co n s i derably more recent
origi n t h a n the ovu m to which t hey a re head i n g.
When sperm enter the vagi n a , t hey beg i n a w i n d i n g
j o u rn ey t h a t takes them t h rough t h e cerv i x , t h e o pe n i n g i n t o
t h e u teru s , and i n t o the fa l l o p i a n t ube, where fe r t i l ization
may take p l ace. H oweve r, o n l y a t i ny fr act i o n o f t h e 300 m i l ­
l i o n cells t h a t a re typica l l y ej acula ted d ur i n g sex u a l i n ter­
course u l t i mately survive the a rd u o u s j o u rney. Tha t 's usually
o kay, though: I t t a kes only one sperm to fe rt i l i ze an ov u m ,
and each sperm a n d o v u m conta i n s a l l t h e ge net ic d a t a nec­
essary to p roduce a new h u m a n .
The Stages of the Prenatal Peri o d :
The O n set of Deve l o p ment
The prenatal period co nsists of three phases: the ge rm inal,
embryonic, and fetal stages. They are s u m m a r i zed i n Table 3 - 5 .
The Germ i n al Stage: Fertil ization to 2 Weeks I n t h e
germ inal stage, t h e fi rst-a n d sho rtest-stage o f t he p re n a ­
tal period, the zygote begins to d iv i d e a n d grow in co m plex­
ity d u r i n g the fi rst 2 weeks fol l ow i n g conce p t i o n . D u r i n g t h e
ge rminal stage, t h e fe r t i l i zed egg ( n ow called a /;/c1:;tocys t )
t ra ve l s towa rd the u terus, wh ere i t becomes i m p l a n ted i n t h e
u terus's wa ll, wh ich is r i c h i n n u t rients. The ger m i n a l stage i s
characterized b y m e t h o d i c a l c e l l division, w h i ch gets o ff to a
q u i c k start: Th ree days a fter fer t i l iza t i o n , the o rgan ism con­
sists o f so m e 3 2 cel ls, a n d by the n e x t day the n u mber d o u ­
b les. Vli t h i n a week, i t i s m a d e u p o f I 00 to 1 50 ce l l s , a n d the
n u m ber rises w i t h in creasi n g rapidi ty.
In addition to increasing in n u mber, the cells of t h e o rga n ­
i s m become i ncreasingly specialized . F o r i nstance, s o m e cells
form a protective layer around the mass of cells, wh ereas others
begin to establish the rud i m e n ts of a placenta and umbil ical
cord . When fully developed, the placenta serves as a conduit
between the mother and fetus, prov iding nou rishment and
oxygen via the wnhilical cord. I n addition, waste m aterials from
the developing child are removed th ro ugh the u m b i l ical co rd .
The Em bryon i c Stag e: 2 Weeks to 8 Weeks By th e
e n d of t h e ger m i n a l period, j u s t 2 weeks a fter concep t i o n ,
t h e organ i s m is fi rm l y sec ured to the wa l l o f t h e mot her's
uterus. At this poi n t , t h e c h i l d is c a l led a n e11Jbryo . The
embryo n i c stage is the period fro m 2 to 8 weeks fo l l o w i n g
fer t i l i za t io n . One of t h e h i g h l i g h t s o f t h i s stage i s t h e devel­
opment of the major o rga n s and basic a n a t o m y.
At the begi n n i n g of t h e e m b r yo n ic stage, t h e develo p i n g
c h i l d has th ree dist i nc t l ayers, e a c h of w h i c h w i l l u l t i m a t el y
fo r m a d i fferen t s e t o f s t r uct u res as deve l o p m e n t p roceeds.
Stages of the Prenatal Period
Germinal (Fertil ization-2 Weeks)
Fetal (8 Weeks-Birth)
Embryonic (2 Weeks-8 Weeks)
The germinal stage is the first and short­ The zygote is now designated an
embryo. The embryo develops th ree lay­
est , characterized by methodical cel l
division and the attachment of the organ­ ers , which ultimately form a different set
of structures as development proceeds.
ism to the wall of the uterus. Three days
The layers are as fol lows: Ectoderm :
after fertil ization, the zygote consists of
Ski n, sense organs, brain, spinal cord;
32 cells, a number that doubles by the
next day. Within a week, the zygote mul­ Endoderm: Digestive system, liver, respi­
ratory system; M esoderm : Muscles,
tiplies to 1 00 to 1 50 cells. The cells
blood , circulatory system. At 8 weeks,
become special ized , with some forming
the em bryo is 1 inch long.
a protective layer around the zygote.
The outer layer o f t h e emb ryo, the ectodcn1 1 , w i l l form skin,
h a i r, tee t h , sense o rga ns, a nd the brain an d spinal cord . The
endoden1 1 , t h e i n ner layer, produces the digest ive system, liver,
pancreas, a n d res p i ra tory system. Sandwiched between the
ectoderm and cndoderrn i s t he 111esoden11, from which the
m uscles, bones, blood and circula tory system are forged.
Every part of the body is fo rmed from these three layers.
If you were looki ng at an embryo at the end of the embry­
o n i c stage, yo u m ight be hard-pressed to iden tify it as human.
Only an inch long, an 8- week-old embryo has what appea r to
be gills and a tai l - l ike stru c t ure. On the other hand, a closer
look reveals several fa m i l i a r features. Rudimentary eyes, nose,
Fetus at 5-6 weeks
The fetal stage formally starts when
the differentiation of the major
organs has occurred. Now called a
fetus, the individual g rows rapidly as
length increases 20 times. At 4
months, the fetus weig hs an average
of 4 ounces; at 7 months, 3 pounds;
and at the time of birth, the average
child weighs just over 7 pounds.
l ips, and even teeth can be recognized, and the embryo has
stubby bulges that will form arms and legs.
The head and brain undergo rapid growth during the
embryo nic period. The head begins to represent a significant
proportion of the e mbryo's size, encompassing about 50% of
its total length . The grow th of nerve cells, called neurons, is
Fetus at 8 weeks
Fetus at 1 2 weel�s
Chvt. p te r 3
The Start of Life: Genetics and Prenatal Devel opment
65
The stage that
begins at about 8 weeks after
conception and continues until
birth
Feta l stage
astonishing: As many as 1 00,000 neu­
rons are produced every minute dur­
ing the second month of life! The
nervous system begins to function
around the 5th week, and weak brain
waves begin to be produced as the
nervous system starts to fu nction
(Lauter, 1 998; Nelso n & Bosquet,
2000 ) .
Th e Fetal Stage: 8 Weeks to
Birth It is not u n til the final period
of prenatal development, the fetal
stage, that the devel oping child be­
comes easiJy recogn i zable. The fetal
stage starts at about 8 weeks after
conception and continues u n t i l bi rth.
The fetal stage formally starts when
A s with adults, there are broad
the d i fferentiation of the major or­
differences in the nature of fetuses.
Some are very active while others are
ga ns has occurre d .
more reserved, and these
Now called a fetus, the developing
characteristic s can continue after birth. child undergoes astoundingly rapid
change. For i nstance, it increases i n
length approximately 2 0 times, a n d i t s p roportions change
dramatically. At 2 months, ap proxima tely one-half of t h e
fetus is what will ultima tely be i t s h e a d ; 5 m o n ths, t h e
h ead accou nts for j ust over one- fo urth o f its total s ize ( see
Figure 3- 13 ). The fetus also substantially increases in weight.
At 4 months, the fetus weighs an average of about 4 ounces; at
7 months, it weighs about 3 pounds; and at the time of bi rth
the average child weighs j ust over 7 pounds.
1 /2
2 months
after
conception
3/8
5 months
after
conception
1 /4
Newborn
F I G U R E 3 - 1 3 Body Proportions
During the fetal period, the proportions of the body change
dramatically. At 2 months, the head represents approxi mately half
the fetus. but by the time of birth, it is one fourth of its total size.
66
PA RT O N E
B e g i n n i ngs
Infe rti l ity The inabil ity to
conceive after 1 2 to 1 8 months
of trying to become pregnant
A developing child, from
weeks after conception unti l
birth
Fetu s
8
At the same time, the develop i n g child is ra pidly becoming
more complex. Orga ns become more d i ffere n t ia ted and tart
to work. By 3 months, for exa m ple, t he fet us swallow and uri­
nates. In add ition, the i n terconnections bet ween t he d i fferent
parts of the body become m o re com plex and i n tegrat ed. Arn1s
develop hands; hands develop fi n gers; fi n gers develo p nails.
As this is happen i ng, the fetus makes i tself known to the
outside world. In the earliest stages of p regnancy, m o t hers may
be unaware t J1at tJ1ey are, in fact, pregn a n t . As t he fetus bec omes
increasingly act ive, however, most mothers certa i n ly take notice.
By 4 months, a mother can feel the movement of her ch i ld, and
several months later, others can feel tJ1e baby's kicks throug h the
mother's skin. In addition to the kicks that alert its moth er to its
prese nce, the fetus ca n t u rn , do somersa ults, cry, hiccu p, clench
its fist, open and close its eyes, and suck its t h u mb.
The bra i n beco mes i ncreas i ngly so p h i s t i cated d u ring
the fetal stage. The two sym metrical left a n d righ t h a l ves of
the bra i n , known as hem ispheres, grow rn p i d l y, and t h e i n te r­
conne ctions between neurons beco m e m o re co m p lex . The
neurons become coated w i t h a n i n s u l a t i n g m a te ria l cal led
myelin which helps speed the transmissi o n of m essag es from
the brain to the rest o f the body.
By the en d of the fetal period, bra i n waves arc p rodu ced tha t
indicate the fetus passes through d i fterent stages of sleep and
wakefulne ss. The fetus is also able to hear ( and feel t h e vibrat ions
of) soun ds to which it is exposed. For i nstance, resea rche rs
_
Antho ny DeCasper and Melanie Spence ( 1 986 ) aske d a gro up of
pregn ant mot hers to read aloud the Dr. Seuss story T'l1e Cnt in
the Hat two times a day during the latter months of preg nan cy.
T�1ree days after tJ1e babies were born, they appeared to recog­
mze the story they had heard, responding more to it th an to
another story that had a different rhythm .
In weeks 8 to 24 following conce p t i o n , hormo ne s are
released that lead to the increas i n g d i ffe ren tiatio n of m ale
and female fet uses. For exa m p le, high levels o f a n d rogen are
produc ed in males that a ffect t h e size of b ra i n cel ls a nd t he
grow th of neural connections , wh ich, some sci e n t ists spec u ­
late '. ultima tely may lead to differenc es i n male a n d ..(e m ale
bra111 stru cture and even later va riations in gen d e r- rela ted
_
behav ior ( Berenbaum & Bai ley, 2003 ; Reiner & Gea r ha rt,
2004; Knickm eyer & Baron- Cohen, 2006 ) .
J ust a s no two adults are al ike, n o two fetuses a re the
same. Although develop ment dur i n g the prenatal peri od
follows the b ro a d pa ttern s o u tl i ned h ere, t h e re a re sign i fi ­
cant d i fferences i n t h e speci fic nature o f i n d ivid ua l fetuse s'
behavio r. Some fetuses are exceed ingly active, wherea oth­
ers are mo re sedentary. ( The more active fetuses will p roba ­
bly be m o re active after b i r th . ) S o m e have rel atively q u ick
heart rates, whereas others' heart rates a re slowe r, w i th the
typ i cal range varying betwee n 1 20 and 1 60 beats per m i n u te
( D i Pi etro et al., 2002; Niederhofer, 2004; Tongsong et al., 2005 ).
Such d i fferences i n fetal behavior are d u e i n part to
genetic cha racteri stics inherited at the moment of ferti l i z a ­
t i o n . Other kinds o f d i fferences, though , a re bro u g h t about
by t he n a t ur e o f the env i ro n m e n t in wh i c h the ch i l d spends
Artificial i nsemi nati on
A process of fertilization in which
a man's sperm is placed directly
into a woman's vagina by a
physician
I n vitro ferti l izati on (IVF)
A proced ure in which a woman's
ova are removed from her
ovaries, and a man's sperm are
used to fertilize the ova in a
laboratory
its first 9 months of l i fe. As we will see, there are numerous
ways in which the prenatal environment of infants affects
their devel opment-i n good ways and bad .
Pregnancy Pro b lems
For s o m e co uples, conception p resents a major challenge.
Let's consider some of t h e challenges-both physical and
eth ical-t hat relate to pregnancy.
I nferti lity Some 1 5% of couples suffer from infertility, the in­
abi lity to conceive after 1 2 to 1 8 months of t rying to become
pregnant. I n ferti.l ity is negatively correlated with age. The older
the parents, the more likely infertility will occur; see Figure 3- 1 4.
In men, i n fertility is typically a result of producing too few
sperm. Use of illicit dru gs or cigarettes and previous bouts of
sexually t ra nsmit ted diseases also increase infertility. For
wo men, the most common cause of infertility is failure to
release an egg thro ugh ovulation. This may occur because of a
hormone i mbalance, a damaged fallopian tube or uterus, stress,
or abuse of alcohol or dru gs ( Pasqualotto et al., 2005; Lewis,
Legato, & Fisch, 2006; Kelly-v\leeder & Cox, 2007 ).
Several t rea t ments fo r infertility exist. Some difficulties
can be co r rected through the use of d ru gs or surgery.
Another optio n may be artificial insem ination, a procedure
i n which a man's sperm is p laced directly into a wo man's
vagi n a by a physi cian. I n some situati ons, the woman's hus­
band p rovides the sperm, whereas in others it is an anonymous donor fr om a sperm bank.
.
I n othe r cases, fertilizati on takes place outside of the
m other's body. In vitro fertilizat ion (IVF) is a procedure in
...
..c:
C:
0
C:
n:J
C:
Cl) �
� ... ....
..c:
u
c..
0, Cl)
C �
n:J ·UJ
V,
-0
-0
0
F I G U R E 3- 1 4
Older Women and R i s ks of
P reg nancy
Not only does the rate of
inferti lity increase as women
get older, but also the risk of
chromosomal abnormality.
(Source: Reproductive Medicine
Associates of New Jersey, 2002)
C:
E
C.
o-
-
QJ
'-'
QJ
a:i
0
25
20
15
10
5
0
'Tm their real child, and you're just afrozen embryo
thingy they boughtfrom some laboratory. "
which a woman's ova are removed from her ovaries, and a
man's sperm are used to fertilize the ova in a laboratory. The
fertilized egg is then implanted in a woman's uterus. Similarly,
gamete intrafallopia n trails/er (GIFT) and zygote intrafallopian
tra nsfer (ZIFT) are procedures in which an egg and sperm or
fertilized egg are implanted in a woman's fallopian tubes. In
IVF, GIFT, and ZIFT, implantation is done either in the
woman who provided the donor eggs or, in rarer instances, in
Preg nancy
�
\:
.&
C:
(I)
2
QJ
1
-"
,.,..
�
o _,___c___--'--------�
1 2-1 9 20-24 2 5-29 30-34 35-39 40-44 45+
Age
1 5 20 25 30 35 40 45 50
Age
60
Misca rriage
5
�
OJ 40
\:
-"' �
QJ
C.
...: 20
ii:
in
V,
u.. -
C:
0
Ectopic p reg nancy
3
II)
·Ct:
....
Cl)
__L__:____;___;_____;___;____:__J
20-24 25-29 30-34 35-39 40-44 45-50
Age
ChliLpter 3
....
C:
Cl)
\:
Cl)
C.
Chromosomal abnormality
4
3
,
2
o_
20
25
30 3 5 40
Age of Mother
45
The Start of Life: Genetics and P renatal Devel opment
67
Teratogen A factor that
produces a birth defect
a surrogate mo ther, a woman who agrees to carry the child to
term. Surrogate mothers may also be used in cases in which
the mother is unable to conceive; the surrogate mother is arti­
ficially insemi nated by the biological father, and she agrees to
give up rights to the infant ( Frazier et al., 2004; Kolata, 2004 ) .
In vitro fertilization is increasingly successful, with suc­
cess rates of as high 33% for younger women (but with lower
rates for older women ) . Furtherm ore, reproductive tech­
nologies are becoming increasingly sophisticated , perm itting
parents to choose the sex of their baby. One technique is to
separate sperm carrying the X and Y chromosome and later
implanting the desired type into a woman's u teru s . In
another technique, eggs are removed from a wo man and fer­
tilized with sperm using in vit ro fertil iza tion. Three days
after fertilization, the emb ryos are tested to determ i n e thei r
sex. If they are the desired gender, they are then impla nted
into the mother (D uenwald, 2003, 2004; Kalb, 2004 ) .
Eth ical I ssues The use o f surrogate mo thers, i n v i t ro fe r­
tilization, and sex selection techniques p resent a web of eth i ­
cal and legal issues, as well as many emotio nal concerns. I n
some cases, surrogate mothers have refused t o give u p the
child after its birth, whereas in others the surrogate mother
has sought to have a role in the child's life. In such cases, the
rights of the mother, the father, the surrogate m other, and
ultimately the baby are i n conflict.
Even more troubling are concerns raised by sex selection
techniques. Is it ethical to terminate the life of an embryo
based on its sex? Do cultu ral press ures that may favor boys
over girls make it permissible to seek medical in tervention to
produce male offspri ng? And-even more d isturbing-if it is
permissible to intervene in the reprod uctive process to obta i n
a favored sex, what abo u t other characteristics determined by
genetics that it may be possible to preselect for in the fut u re?
for instance, assuming the technology advances, would it be
eth ical to select for a favored eye or hair color, a certain level
of intelligence, or a particular ki n d of personality? That's not
feasible now, but i t is n ot out the realm of possibility in the
future ( Bonnicksen, 2007; Mameli, 2007; E. Ro berts, 2007 ) .
For the moment, many o f these ethical issues rem ain
unresolved. But we can an swer one question: How do ch i l ­
dren conceived using emerging reproductive tech nologies
such as in vitro fertil ization fare?
Research shows that they do q u i te wel l . In fact, some
stu d i es fi nd that the qual ity of fam ily l i fe for those who have
used such techn iques may be superi or to that i n fa m i l ies
with n aturally conceived ch ildren. Furthermore, the later
psych ological adj ustment of children co nce ived usi ng in
vitro fert i l ization and artificial i nsem inat i o n is no d i fferent
fro m that of c h i l d ren con ceived using natural tech n iques
/ Hahn & Di P ietro, 200 1 ; Gol umbok et al., 2004; D i P i et ro,
Costiga n , & Gurewi tsch , 2005; Hjel msted t, Widstro m , &
CoJ l i n s, 2006 ) .
O n the other h a n d , th e i n creasing use o f ! V F tech n iq ues
by older i n d i v i d u a l s ( who m ig h t be q u i te elderly when t h e i r
68
PA RT O N E
Beg i n n i n g s
ch i ldren reach adolesce nce ) may cha nge t hcse posi t i vc fi nd­
i ngs. Because widespread u se o f 1 \/F is only recen t , we j ust
don't know yet what will happen w i t h agi ng parents ( Col p i n &
Soenen, 2002 ) .
Miscarriage a n d Abortion A 1 1 1 isrnrringe-known a s a
sponta neous abort ion-occu rs when p regn a nq' ends before
the developi ng chi l d is able to s u rvive ou tside t h e mot her's
womb. The emb ryo det aches fr om t h e wa l l n f t h e u t e r u s and
is expelled.
Some 1 5% to 20% of all p regna n c ies end in m i scarr i age,
usual ly i n the fir st several m o n t h s o f p regnan cy. Many occur
so early that the mother is not even aware she was preg n a n t
a n d may n o t even know s h e h a s suffe red a m i sca r r iage. Ty p ­
ically, miscarriages are a t t r i b u table t o some sort o f genetic
abnormality.
I n abortion, a mother vol u n tari l y ch ooses to t e r m i n ate
pregnancy. I nvolvi ng a complex set of physical, psychological,
legal, and ethical issues, abortion is a d i ffi c u l t choice fo r every
woman. A task force of the American Psychologica l Associa­
tio n ( A PA ) , wh ich looked at the aftereffects of abortion,
fo u n d that, fol lowi n g a n abort ion, most wo men experienced
a combin ation of rel ief over term i nating an u nwa n ted p reg­
nancy, and regret and gu i l t . However, i n most cases, the nega­
tive psychological aftere ffects d i d not las t, except for a small
proportion of women who al ready had serious emot ional
problems ( A PA Reproductive Choice Worki ng G ro u p, 2000 ) .
Other research fi n ds that abortion may b e associa ted
with an increased risk of fu t u re psychological p roblems.
H owever, the fi nd i ngs a re m i xed, and th ere a re sign i fica n t
ind ividual differences i n how wo men respond t o the experi­
en ce of abortion. What is clear is t h a t i n all cases, abort ion is
a difficult decision ( Ferguss o n , Horwood, & Ridder, 2006) .
environ mental age n t such a s a dru g, chemical, virus, o r other
factor that produces a bi rth defect. Although it is the job of
the placenta to keep teratogens from reach ing the fetus, the
placenta is not en t i rely successfu l at this, and probably every
fetus is exposed lo some teratogens.
The t i m i n g a n d q u a n t ity of exposure to a teratogen are
cru cial. At o m e ph ases of prenatal devel opment, a certain
teratogen may have only a m i n i mal impact. At other periods,
however, the same tera togen may have profound conse­
quences. Genera lly, teratogens have their largest effects d ur­
i ng periods of especially rapid prenatal development.
Sensitivity to specific teratoge ns is also related to racial and
cultur a l backgro u nd. For exam ple, Nat ive American fetuses
are more susceptible to the effects of al cohol than those of
European American descent ( Kinney et al., 2003; \.\l inger &
Woods, 2004 ) .
Furthermore, d i fferent organ systems are vu lnerable to
teratogens at d i fferent t i mes during develo pment. For exam-
pie, the brain is most susceptible 1 5 to 25 days after concep­
tion, whereas the heart is most vulnerable 20 to 40 days fol­
lowing conception (see Figure 3- 1 5; Bookstein et al ., 1 996;
Pakjrt, 2004) .
We will consider t h e findings relating t o specific terato­
gens next. As we do, we keep in mind the broader social and
cultural conte;,..i: in which teratogen exposure occurs. For
example, living in poverty increases the chances of exposure
to teratogens. Mothers who are poor may not be able to
afford adequate diets, nor may they be able to afford adequate
medical care, making them more susceptible to illness that
can damage a developing fetus. They are more likely to be
exposed to pollution. Consequently, it is important to con­
sider the social factors that permit exposure to teratogens.
Mother's Diet Most of our knowledge of the enviro n ­
mental factors that affect the developing fetus comes from
the study of the mother. For instance, as the midwife pointed
The Prenatal Envi ron m ent:
Threats to Development
Acco rd ing t o the Siriono people o f South America, i f a p reg­
nant woman eats the meat of cer tain kinds of a n i m a l s , she
runs the risk of having a ch i l d who may act and look l i ke
those animals. According to o p i n i o n s offe red o n dayt ime tel ­
evision talk, a pregnant mother should avo id getting a ngry
in order to spare her c h i l d fr om entering the world with
anger ( Cole, J 992 ) .
Such vi ews are l a rgely the stuff o f fo lkl o re, a l t hough
there is some evidence that a m o t her's anxiety d u ri n g preg­
nancy may affect the sleep ing patterns of the fetus prior to
birth . There are certa i n aspects of a mothers' a n d fa thers'
beh avior, both before and after concep t i o n , that can produce
l i fel o n g conseq uences for t h e ch i l d . Some conseq uences
show up i m mediately, but half the possible p ro b l e m s a ren't
appare n t before birth. Other p roblems, more i n sidious, may
not appear u n t i l yea rs after b i rt h ( G ro o m e et a l . , 1 995;
Couzin, 2002 ) . Some of th e most pro f o u n d co nsequences
a re bro u g h t abo u t by tera toge n i c agen ts. A te ratogen is a n
Depending on their state of development, some parts of the body vary in their sensitivity to teratogens.
(Source: Moore . 1 9 74)
Cnctp te r 3
The Start of Life: Genetics an d Prenatal Development
69
Feta l a l c o h o l syndrome (FAS)
A disorder caused by the
pregnant mother consuming
substantial quantities of alcohol
during preg nancy, potentially
resulting in mental retardation
and delayed growth in the child
out in our earlier example of Lisa and Robert, a mother's d iet
The ri sks i nvolved in pregn ancy arc grea ter not only for
clearly plays an important role in bolstering the develop­
older mothers but also for atypically you n g women. Wo men
ment of the fetus. A m o ther who eats a varied diet h igh i n
who become pregnant during adol esce nce-a n d such preg­
nutrients i s a p t t o have fewer compl ications du ring pregnan­
nancies actually encompass 20% of a l l p regnancies-a r more
cy, an easier labor, and a generally healthier baby than a
likely to have premature deliveri es. Fu rt hermore, t h e mortality
mother whose diet is restricted in nutrients ( Kaiser & A l l e n ,
rate of i n fa nts born to adolescent m o t hers i · double t ha t for
2002; Guerrini, Thomso n , & Gurling, 2007 ) .
mothers i n their 20s ( Ki rchengast & 1-I a rt man n , 2003 ) .
The problem of diet i s of i mmense global concern, with
(
800 million h ungry people i n the world. Even worse, the
Mother's Prenatal S u p port Keep i n m i n d , t h ough , t h a t
number of people vulnerable to h unger is close to 1 billion.
t h e h igher mortal i t y r a t e fo r b a b i e s o f adolesce n t mot hers
Clearly, restrictions in diet that bring about hunger o n such
reflects more than j ust physiol ogical p ro b l e m s rel a t e d t o the
a massive scal e affect mill ions of ch ildren born to women
mothers' yo u n g age. You ng m o t hers o ften face ad verse social
l iving under such conditions ( Un ited Nati ons, 2004) .
and economic factors which can a ffect i n fa n t h ea l t h . Many
Fortunately, there are ways to counteract the types of
teen age m o t hers do n o t have enough money o r soc ial sup­
maternal malnourishment that affect prenatal developm e n t .
port, a s i t uation that p revents t h e m from get t i ng goo d pre­
Dietary s upplem ents given t o moth ers can reverse s o m e o f
natal ca re and pare n t i ng support a fter t h e baby i s born.
the problems produced b y a poor diet. Fur thermore,
Pove rty o r social c i rcu msta nces, such as a lack o f pa ren tal in­
research shows that babies who were malnou rished as
volveme n t or supervision may eve n have set t h e stage fo r the
fetuses, but who are subsequently raised i n enriched envi­
adolescen t to beco me p reg n a n t i n the fi rst p l ace ( D i Piet ro,
ronments, can overcome some of the effects of their early
2004; H u i zi n k , Mu lder, & B u i t e l a a r, 2004 ) .
malnourish ment. However, the real ity is that few o f the
wo rld 's children whose mothers were malno urished befo re
M other's H ealth M o t h e rs who eat t h e r i g h t foods, m a i ntheir bi rth are apt to find themselves i n enri ched environ ­ , t a i n an acceptable weight, and who exe rc ise a p p r opriat e! }'
ments after birth ( G rantham-McG rego r et al., 1 994; Kramer,
maxim ize the chances of having a heal thy baby. Fu r t her­
2003; Olness, 2003 ) .
more, they can red uce t h e l i feti m e risk of obes i t y, h i gh bl ood
press u re, and heart d i sease i n t h e i r ch i l d re n by m a i n t a i n i n g a
Mother's Age More women are giving birth later i n l i fe
healthy l i festyle ( Wa l ker & H u m p h r i es, 2005, 2007 ) .
than was true j ust 2 or 3 d ecades ago. The cause for t h i s
I n co n t rast, i l l ness i n a p regn a n t wo m a n can have d evas­
change i s l argely due t o transformations in society, as m ore
tating conseq uen ces. Fo r i n sta n ce, the o nset of rrrudln ( Ger­
women ch oose to conti nue th eir education with adva nced
man measles ) i n the mother prior to the I 1 t h week of
degrees and to start careers p rior to givi ng bi rth to their fi rst
pregna ncy is likely to cause serious co nsequences i n t h e
chi l d (Gibbs, 2002; Wildberger, 2003; Bornstein et al . , 2006 ) .
baby, including b l i n d n ess, dea fness, heart defects , o r b ra i n
Consequently, the number o f women wh o give birth i n
damag e. In later stages o f a pregnancy, howeve r, a dvers e con ­
their 3 0s and 40s has grown considerably si nce the 1 970s.
seq uences o f rube l l a beco m e i n c reas i n g l y less l i ke l y.
However, this delay in childbirth has potential conseq uences
Several other d iseases may a ffec t a devclo p i n g fetus,
for both mothers' and children's heal th. Women who give
agai n depen d i ng on wh en the i l l n ess i s co n t ra c t e d . Fo r
birtI1 when over the age of 30 are at greater risk for a va riety
instanc e, chicken pox may produce b i rt h defec ts, whereas
of pregnancy and birth complications than are younger
m u mps may increase the risk of m i scarr iage.
mothers. For i nstance, they are more apt to give bi rth prema­
So me sexu ally t ra n s m i t ted d i s eases such a s syp h ilis ca n
turely, and their children are more likely to have low b i rth
be passed directly t o t h e fetus, who w i l l be bo r n s u ffer i ng
weights. This occurs in part because of a decline i n the cond i ­
fro m the d i sease. In some cases, sex u a l l y t r a n s m i tted d i sease s
tion o f a wom an's eggs. For example, b y t h e t i m e they a re 4 2
such as gon o rrhea a re com m u n i cated to t h e c h i ld as i t passes
years old, 9 0 % of a woman's eggs are no longer normal
t h rough t h e birth canal to be b o rn . A cq u i red i111 1 1 1 1 1 11e de/i­
(Cnatti ngius, Berendes, & Forman, 1 993; Gi bbs, 2002 ) . Older
cie11 cy syndrome ( A IDS ) is t h e n ewest of t h e d i seases to a ffect
moth ers are also considerably more l i kely to give birth to
a newbo rn . Moth ers who h ave the d isease or who m erel y are
children with Down syndrome, a form of mental retardat i o n .
ca rriers of the v i r u s may pass i t on to t h e i r fet u ses t h ro ugh
About 1 o u t o f I 00 babies born t o m others over 4 0 yea rs o f
the blood that reaches the p l ace n t a . However, if m others
age h a s Down syndro me; for mothers over 5 0 , t h e i n c i dence , with A I D S a re t reated w i t h a n t ivi ral d ru gs such as AZT d u r­
i n c reases to 25%, o r I i n 4 ( Gaulden, I 992 ) . O n the other
i n g pregn a n cy, less than 5% o f those i n fa n ts a re b u rn w i t h
hand, some re earch shows that older mothers a re not auto­
t h e d i sease. I n fa n ts w h o are bo rn w i t h A I DS m us t rem a i n o n
matica l l y at risk for m o re pregnancy p robl ems. For instance,
a n t iv i ral d r ugs t he i r e n t i re l i ves ( Nes h e i m et a l . , 2004 ) .
one study fou n d that when women i n their 40s who had n o t
experien ced health d i ffi c u l t i es were co n s i dered, they were n o
Mothers' Drug U se Mothers' use o f many k i n d s o f
more l i kely t h a n women i n thei r 2 0 s to have prenatal prob ­
d rugs-both legal and i l l ega l-poses s e r i o u s r i s ks to t h e u n ­
l e m s ( A les, Druzi n , & Sa n t i n i , 1 990; D i l d y et a l . , 1 996 ) .
b o r n c h i l d . Eve n ove r - th e - co u n t e r remed i es fo r co m m o n
70
PAR T O N E
Beg i n n i ngs
Feta l a l c o h o l effects ( FAE)
A cond ition in which children
display some, although not all, of
the problems of fetal alcohol
syndrome due to the mother's
consumption of alcohol during
pregnancy
ailment can have su rprisi ngly i njurious conseq uences. For in­
stance, aspi r i n taken fo r a h eadache can lead to fetal bleeding
and growth i m pa i rments ( G r i ffith, Azu ma, & Chasnoff, 1 994 ) .
Eve n d rugs p resc r i bed by medical pro fessionals have
som e t i mes h ad d isast ro u s conseq uen ces. I n the l 950s, m a ny
wo men who were told t o t a ke 1!,o/irlo111irle for m o rn i n g sick­
ness d u r i n g t h e i r p reg n a nc ies gave birth to children w i t h
s t u m ps i nstead o f a n m and legs. Alt hough the physicians
who p resc ribed the drug d i d not know it, thalido m i d e inhib­
ited the grow t h o f l i mbs that normally wou l d have occurred
during t h e fi rst 3 m o n t h s of pregnancy.
Some d ru gs ta ken by mothers cause d i fficulties in their
children l iterally decades after t I1ey were taken. As recently as the
1 970s, rhe art i ficial hormone rliethylstiluestrol ( DES) was fre­
quently prescribed to prevent miscarriage. Only later was it
found that the da ughters of mothers who took DES stood a
much h igher than n�rmal chance o f developing a rare form of
vagi nal or cervical cancer and had more di fficulties during their
pregnancies. Sons of the mothers who had taken DES had their
own p roblems, includ i ng a h igher rate than average of repro­
ductive difficulties ( Schechter, Finkelstein, & Koren, 2005 ) .
B i r t h con t ro l or fert i l i ty p i l l s taken b y pregna nt women
before they arc aware o f their p regnancy can also cause fetal
da m age. Such medicines contain sex hormones that affect
developing bra i n s t ru c t ures in the fetus. The se hormones,
wh ich when prod uced naturally a re rela ted to sexual differenti­
ation i n the fetus and gender differences after birth, can cause
sign ificant dam age ( M iller, 1 998; Brown, H ines, & Fane, 2002 ) .
I l l ic i t d r u gs m ay pose equally great, a n d sometimes even
greater, r i sks for the enviro n ments of pre natal childre n . For
one t h i ng, the p u r ity of d ru gs purchased il legally varies sig­
n ifi cantly, so drug users can never be qui te s ure what speci f­
ical ly they are i ngest i ng. Furthermo re, the effects of some
commonly used i l l ic i t drugs can be particu larly devastatin g
( Mayes et a l . , 2007 ) .
Consider, fo r instance, the use o f 111a rij11a11n; i t is cer­
tainly one of the most co m m o n ly used illegal d ru gs-mil­
l ions o f people i n the United States have admi tted trying it.
Marij u a na used d ur i ng pregnancy can restrict the o>..-y ge .n
that reaches t h e fetus. Its use can lead to i n fants who are nTt ­
table, nervo us, a n d easily disturbed. Children exposed t o
marijuana p renatally show learning and memory deficits at
the age of 10 ( Huizink & Mulde r, 2006; Sm ith et al., 2006;
Wi lliams & Ross, 2007 ) .
During the ea rly l 990s, cocni,,e use by pregnant women
led to an epidem i c o f thousands of so-called "crack babies."
Coca i ne produces an intense restriction o f the arteries leadi ng
to the fetus, ca using a significant reduction in the flow of blood
and oxygen, i ncreasing the risks of fetal death and a n u mber of
birt h defects and disabilities ( Schuetze, Eiden, & Coles, 2007 ) .
Children whose mothers were addicted to cocaine may
themselves be born addicted to the drug and may have to suffer
through the pain of ,vithdrawal. Even if not add icted, they may be
born with significant problems. They are often shorter and their
weight is less than average, and they may have serious respiratory
problems, visible birth defects, or seizures. They behave quite dif­
ferently from other infants: Their reactions to stimulation are
muted, but once they start to cry, it may be hard to soothe them
(Singer et al., 2000; Eiden, Foote, & Schuetze, 2007) .
It is difficult to determine the long-term effects of mater­
nal cocaine use in isolation, because such drug use is often
accompanied by poor prenatal care and impaired nurturi ng
following birth. In fact, i n many cases it is the poo r caregiving
by m others who use cocaine that results in children's prob­
lems, and not exposure to the drug. Treatment of children
exposed to cocaine consequently requires not only that the
ch ild's mother stop using the dru g but also a positive
i m p rovement in the level of infant care that the mother or
other caregivers provide. ( Brown et al . , 2004; Schempf, 2007) .
M others' Use of Alcohol and Tobacco A pregnant
woman who reasons that having a drink every once in a while
or smokin g an occasional cigarette has no appreciable effect
on her unborn child is, in all l ikelihood, kidding herself: In­
creasing evidence su gg ests that even small amounts of alco­
hol and n icotine can disrupt the development of the fetus.
Maternal use of alcohol can have profound conse­
q uences for the unborn child. Alcoho lics who consume sub­
stantial quant ities of alcohol during pregnancy, place their
children at the greatest risk. Ap p roximately 1 out of every
750 i n fants in the United States is born with fetal alcohol
syndrome ( FAS) , a disorder that may include below- average
intell igence and sometimes mental retardation, delayed
growth , and facial deformities. FAS is now the primary pre­
ven table cause of mental retardation ( S teinhausen & S p ohr,
1 998; Burd et al., 2003 ; Calhoun & Warren, 200 7 ) .
Even mothers who use smaller amounts of alcohol dur­
ing pregna ncy place their ch ild at risk. Fetal alcohol effects
( FAE) is a condition in which children display some,
altho ugh not all, of the problems of FAS due to their
mothe r's consu mption of alcohol during pregnancy ( Baer
et al., 2003 ; Molina et al., 200 7 ) .
Children who do n o t have FAE may sti l l b e a ffected by
their mothers' use of alco hol. Studies have found that mater­
nal consumption o f an averag e o f just two alcoholic drinks a
day durin g p regna ncy is associated with lower intelli gence i n
their o ffsp ring at age 7 . Other research concurs, suggesting
that relatively small quantities of alco h o l taken d urin g preg­
na ncy can have future adverse effects on children's behavior
and psychological fu nctioning. Furthermore, the conse­
quences of alcohol ingestion during p reg na ncy are long last­
ing. For example, one study fo u nd that 1 4-yea r-o lds' success
on a test i nvo lving spatial and visual reasonin g ,,v as related to
their mothers' a lcohol consum p t i o n d urin g pre g nancy. The
more the mothers r ported d r i n ki n g , the less accurately their
chi ldren responded ( Johnson et al. , 200 l ; Lynch et a l . , 2003;
Mattson, Calarco, & La ng, 2006 ) .
Because o f t h e risks asso c i a ted w i t h a l c o h o l , phys i ­
c i a n s today counsel p reg n a n t wo m e n ( a n d even t h o s e
w h o a r e t ry i n g to beco m e p regn a n t ) to avo i d d r i n k i ng a n v
ChCl.p te r 3
The Start of Life: Genetics and Prenatal Develo pment
71
alcoholic beverages. In additio n , they c a u t i o n aga i nst
smoking-an other practice p roven to h ave an adverse
effect o n an unborn child. Smoking produces seve r a l c o n ­
sequences, n o n e go od. F o r sta rters, smoking reduces t h e
oxygen content a n d increases t h e carbon m o n ox i d e i n t h e
mother's b l o o d , which quickly reduces t h e oxyge n a va i l ­
a b l e t o th e fetus. I n additio n , the nicotine a n d o t h e r tox­
ins in cigarettes slow the res p i ration rate of t h e fet u s and
speed up i t s heart.
The ultim ate res ult is an increased possibility o f m iscar­
riage and a higher likelihood of death d u ring infancy. I n fact,
estimates s uggest that smoking by p regnant wo men leads to
more than 1 00,000 miscarriages and the deaths of 5,600
babies in the United States alone each year ( Hasl a m &
Lawrence, 2004; Triche & Hossain, 2007 ) .
Sm okers are two times a s l i kely a s non s m okers t o have
babies with an abnormally low bi rthweight, and sm okers'
babies are shorter, o n average, than those of n o n smokers.
Furth ermore, women who smoke during pregnancy are
50% m o re likely to have m e ntally reta rded c h i l d ren.
Finally, m others who s m o ke a re m o re l i kely to h ave c h i l ­
dren who exh i b i t disruptive behav i o r d u r i n g c h i l dhood
( D rews et a l . , 1 99 6 ; D ej i n - Karlsson et al., 1 9 9 8 ; Wa ksc h a l g
et a l . , 2006 ) .
The consequences of smoking are s o p ro fo u n d that i t
may affect n o t o nly a mother's children, b u t h e r grandch i l ­
dren. F o r example, children whose grandmoth ers smoked
during pregnancy are m ore than twice as likely to develop
childhood asth ma than are children o f grandmothers who
d i d not smoke ( L i et al. , 2005 ) .
D o Fat h e rs Affect t h e P re n atal E n v i ro n m e nt? It
wo u l d b e easy to reason that once the fa t h e r has d o n e h i s
p a rt in t h e sequence o f events l ea ding to co n c e p t i o n , h e
would h ave n o role in t h e prenata l environment o f the
fetus. In the past, develo p m ental res e a rchers h ave general­
ly sh ared th i s vi ew, and there is relatively little research i n ­
vestigatin g it.
However, it is becoming increasingly clear that fathers'
b ehavi o r m ay in deed i n fluence the prenatal environment.
Consequently, health practition ers are utilizing avail­
able research to suggest ways fathers can s u p p o rt
healthy prenatal development, as ou r story of Lisa
and Robert's visit to the midwife i l l ust rated.
Prov i de r's
For instance, fathers- to-be should avo id
p ers pec tive:
smoking. Secondhand smoke fro m a father's cig­
Ir arJ r 1t1o r , t0 avr_,1d1 g
arettes may affect the mother's health, which in
srrn-_, k1ng !!ha ".Jther
turn influences her unborn child. The greater the
sr,rts 'J rr,ir-19s might
level of a father's smoking, the lower the birth­
rrnr,r:; rs - tr..,- 1:;<:, d':J tr,, nelr-,
weight of h is children ( Hyssaelae, Rautava, &
1he1r x1r./::irr
Heleni us, 1 995; Tomblin, Hammer, & Zhang,
-.r 111r:Jri:;r, ':Je1':Jl1Jµ
1 998 ) .
Simila rly, a fath er's u s e of alcohol a n d i l l egal
drugs can have significant e ffects o n the fetus. Alco­
h o l a n d drug u se i m p a i r s sperm and may l ead to
f
72
PA RT O N E
B eg i n n i n g s
ch romosomal damage that may affect the fe t u a l conception. ln
addition, alcohol and drug use during p regna ncy may also affect
the prenatal environ ment by c rea t i n g st ress i n t h e mot her and
ge nerally prod ucing an u n healthy env i ro n m e n t . A father's expo­
sure to environmental toxins i n the wo rkp lace, such as lead or
mercury, ma}' bind themselves to sperm and cause birth defect
( Wakefield et al ., 1 998; Dare et a l . , 2002; Choy ct a l . , 2002 ) .
Finally, fathers w h o are phys ica l l y or e m o t i o n a l l y abusive
to their pregnant wives can damage t h e i r u n b o rn c h i l d ren. By
increasing the level of maternal st ress, or act u a l l y causi ng physi­
cal da mage, abusive fathers i n c rease the risk o f h a rn1 to their
unborn children. In fa c t , 4% t o 8% of wo men face physical
abuse during pregnancy ( Gazmarian et a l . , 2000; Bacchus,
Mezey, & Bewley, 2006; M a r t i n et -,I . , 2006 ) .
B eco ming a.n I nfo rmed
Co ns ume r of D eve l o p me nt
•
Mon itor caffei n e i ntake. Although it is sti ll unclear whether
caffe ine produces b i rth defects, it is known that the caffeine
found i n coffee , tea, and chocolate can pass to the fetus,
acting as a sti m ulant. Because of thi s, you probably should­
n ' t dri nk more than a few cups of coffee a day (Wisborg et
al. , 2 003) .
R EV I EW
l . vVhen sperm enter the vagina, they travel thro ugh the
cerv ix. and i n to the fallopian tube where ____ may
take place.
UO!JEZ!f!l.IJJ :JJ.M.SUV
• Whether preg n ant or not, don't smoke. This holds true for
mothers , fat hers , and anyone els e in the vicin ity of the preg­
nant mother, because research suggests that smoke i n the
fet al environment can affect birthweig ht.
•
.J
2. A ___ occurs wh en pregnancy ends before the developing child is able to survive outside the mother's wo mb.
JEE\.l.lEJS!lU :Jc>.M.SUV
environmental
age
n
t
such
as
a
drug,
3. An
chemical, virus,
o r other factor that produces a birth defect is called a
Exerc i s e reg u l arly. In most cases, women can continue to
exercise, particula rly exercises i nvolving low-impact rou­
tines. On the other h a n d , extreme exercise should be avoid­
ed, especially o n very hot or very cold d ays . "No pai n , no
gain" isn 't applicable d u ring pregnancy (Paisley, J oy, & Price,
•
2003 ; Schmidt et al . , 2 006) .
uc>l3oie1;,J :.1a.M.suy
� Ta see more review q 11estio1 1s, log on to /vfyDevelopmen tLa b.
Opti m izing the Prenatal Envi ron ment
If you are contemp lating ever h aving a chi l d , yo u may be over­
w hel med , at this point in the c h a pter, by t h e n u m ber of things
that can go wron g . Don't be. Although both genetics and the
environ ment pose their share of risks, in the vast major ity of
case s , p reg na ncy and birth proceed without m i s h a p . M o reov er,
there a re several t h i n g s that women can do-bo t h be fore and
d u ring preg nan cy-to optimize the p robabil ity that pre gn ancy
will p rog ress smoothly (Massaro , Rothbau m , & Aly, 20 06) .
Amon g the m :
•
•
For women who are p l a n n i n g to become p reg n a n t , s evera l
p reca utio n s a re in o rd e r. Fi r st , wome n s h o u l d h ave n o n ­
emerge ncy X- ray s o n l y d u r i n g t h e f i r s t 2 weeks afte r their
menstru al p e ri o d s . Seco n d , women s h o u l d b e va cci n ated
a g a i n st rubel l a (German measl es) at l east 3 a n d prefe rabl y
6 mo nths b efore gett i n g preg n a n t . F i n a l ly, w o m e n who are
p l a n n i n g to b ecome pregn ant s h o u l d avo i d t h e u se of birth
co n tro l p i l l s at least 3 m o n t h s before try i n g to c on ceiv e,
be cause of disruptions t o h o rmonal p ro d u c t i o n cau se d by
th e p i l ls .
Eat w el l , both before and d u ring (and after, for that m atter!)
pregnancy. Pregnant mothers are , as th e sayi ng goes, eating
for two. This means that it i s more essential th a n ever to eat
regular, wel l- balanced meals . I n addition , physicians typically
recommend taki ng prenatal vitam i n s which include fo lic
acids, which can decrease the likelihood of birth defects
(Amit ai et al . , 2004 ).
• D o n ' t u s e a l c o h o l a n d other d r u g s . T h e e v i d e nc e i s clea r
t h at m a n y d ru gs pass d i rectly to t h e fet u s and may cause
b i rt h d efects . I t i s also c lear t h at the m o re on e d r i n k s , the
g reater the ri s k to t h e fet u s . T h e best a d v i c e , w h et h er y o u
are a l ready pre g n a n t or p l a n n i n g to h ave a c h i l d : Do n 't
u s e any d r u g u n l e s s d i rected by a p h y s ic i a n . I f you are
p l an n i n g t o get p reg n a n t , e n c o u ra g e your p a rt n e r to
avo i d using a l c o h o l o r other drugs too ( O ' C o n n o r & Wha­
ley, 2 0 0 6) .
CASE STU DY
T h e C a s e of . . . T h e G e n et i c F i n g e r of Fat e
Melindah and Jermain Tessel were incred i bly happy last week
when they learned that M e l i n d ah was pregnant with their first
c h i l d , but now they ' re so worri ed they can 't sleep.
When they got home from the physican 's visit, they began _ to
o
i
j k n gly consider such characteristics as height (tal l l i ke _ Melm ­
dah or on the short side, l i ke Jermain) , tendency to obesrty (like
Jermain) , ath l etic abil ity (like Melindah ), intell igence (h i g h , of
cou rse, l i ke both of them), and so o n . But then they turned to
other traits.
Even as adu lts both Melinda h and J ermai n are overly shy
and quiet, and they wish they were more assertive . Neither is a
natural leader or confiden t public speaker, but they want their
child ren t o be. Both were loners when they were younger, and
they agree that their kids wou ld have an easier time if they
turn ed out to be more sociab le and outgoing. They worry
whether these personal ity traits are prede termined , o r if their
kid s' fates can be d i fferent.
Later, the conversat ion got even more u nsettling. Melind ah
remem bered that there was some mental illness in her family
and th ere were even rumors of violent behavior by one of her
uncles. This prompted Jermain to recall an alcoholic cousi n and
a more d i stant relative who, he thought, had d i ed early from
sickle-cell anemia.
There seems to be so many th i n g s that cou ld go w rong-al l
because of the baggage they carry in their genes!
1 . How would you beg in to reass u re Mel indah and J er­
main about their worries?
2 . Which c haracteristics that they discussed are largely
genetic, and which are more e nvi ronmentally influ­
enced? Are the geneti c traits equ ivalent to fate, or can
their expression be modified? Why or why not?
3. How much should M e l i ndah worry about the m ental i l l ­
ness a n d violence in h e r fam i ly ? What w ould yo u tell
her?
4. H ow much sho uld Jermain worry about his c hildren in­
heriting sickle-cell anemia?
5. Wou ld you advise Meli ndah and Jermain to s eek ge­
netic counseling? Why or why n ot? What factors wou ld
you consider i n advisi n g them to visit or
not to visit a counselor?
C h. titpter 3
The Start of Life : G e n etics and Prenatal Development
73