The Genetics of Child Development
I. GENETIC FOUNDATIONS
A. The Genetic Code
l. Chromosomes: "colored bodies" carrying
genetic material
contained in the nucleus of all cells except
red blood cells
Humans have 23 pairs (n = 46)
Chimpanzees have 24 pairs
Horses have 32 pairs
Mice have 20 pairs
2. Genes: multiple genes per chromosome;
most genes lead to production of some
protein. Genes composed of:
1
3. DNA (deoxyribonucleic acid): genes differ
in length of the segments of DNA
--Double helix structure (twisted ladder);
uncoiled set of 23 chromosomes would
be 3 feet long (in each cell)
--Sides of ladder: alternating sugar and
phosphate molecules in two threads
wound around each other.
--Ladder rungs consist of pairs of
nucleotides (nitrogen-based molecules)
attached to the sugar units of the sides:
Adenine (A) paired with Thymine (T)
Cytosine (C) with Guanine (G)
--sequence of pairings determines the
genetic instructions
--chromosome 1 has 263 million bases;
smallest (chrom. 21) has 50 million
bases (Human Genome project)
2
4. Karyotype: depicts homologous pairs
(except for the XY pair in males)--in
humans, 22 of the pairs are known as
autosomes, and l pair are sex
chromosomes.
5. Gametes: sex cells (egg, sperm)
contain only 23 chromosomes each
6. Mitosis: process by which DNA duplicates
itself
--DNA ladder splits down the middle,
leaving each exposed base free to pick up
its complementary mate from the cell
cytoplasm
--sister
chromatids
attached
at
centromere then separate during cell
division
e.g., Zygote (fertilized egg) replicates in
first 24 hours
3
7. Meiosis: process by which gametes, or
sex cells are formed
--Cell with 46 chromosomes replicates
itself (but doesn't split from sister
chromosome)
--Crossing Over: pairs of chromosomes
exhange corresponding segments to
create new genetic combinations
--Cell then divides, then divides again to
leave 4 cells with 23 chromosomes each
8. Alleles: the different forms/versions of
each gene (e.g., Brown-eye, blue-eye,
green-eye, etc.)
--occur at the same locus on the
autosomes, one each from mother and
father
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9. Genotype: one's genetic inheritance
(e.g., BB or Bb or bb for eye color)
homozygous: having same two alleles
for a trait (e.g., BB or bb)
heterozygous: having two different alleles
for a trait (e.g., Bb)
10. Phenotype: one's expression of a trait
(e.g., Brown eyes vs. blue eyes)
11. Monozygotic Twins share a genotype
(mono- = one zygote); dizygotic (fraternal)
twins do not
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II. GENETIC INHERITANCE
A. Autosomes:
1. Dominant Traits: supercede expression of
recessive traits
2. RecessiveTraits: Exhibited only when
inherit two alleles (only one allele makes one
a carrier)
Dominant
Recessive
Brown eyes
Gray, green, blue
Curly hair
Straight hair
Brown hair
blond or light hair
Non-red hair (BR,bl) Red hair
thick lips
thin lips
dimples
no dimples
farsightedness
normal vision
Rh-positive blood
Rh-negative blood
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3. Co-dominance: With some traits, a
combination of the alleles is phenotypically
expressed, as with AB blood.
Dominant
Recessive
Type A blood
Type O
Type B blood
Type O
4. Polygenic inheritance: More than one
gene influences expression of a trait, such
as intelligence and height
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5. Dominant & Recessive disorders:
a. Dominant
--Huntington's chorea: occurs 35-40
years of age, causes brain
deterioration loss of motor control,
memory, personality, etc.
--Familial Alzheimer's
--Marfan's syndrome
b. Recessive
--Albinism
--Congenital deafness
8
--PKU, or phenylketonuria (chrom. 12)
1 in l0,000 births
lack the ability for the liver to
produce the enzyme phenylalanine
hydroxylase, that converts a
amino acid phenylalanine into
tyrosine
toxic excess of phenylalanine
builds up in nervous system,
leading to symptoms by 3-5
months; death by 4 without
dietary restrictions
phenylalanine is amino acid in
protein, and is found in milk
products, eggs, meat, poultry,
legumes, nuts, wheat & oats
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--Galactosemia (chr. 9, 17, 1—depends
on enzyme missing)
lack enzyme to convert galactose
(from lactose in milk) into glucose.
Buildup in tissues causes mental
retardation, cataracts, enlarged liver,
& kidney failure
Detection prenatally + newborn
--Cystic Fibrosis (chrom. 7)
most common genetic disease
among Caucasians (1 in 3000;
4-5% of Americans are carriers; caused by
a change in single nucleotide, or one letter
error)
respiratory tract becomes clogged
with mucus, increases risk of
infection (death by mid-20s)
secretions also obstruct pancreas,
leading to dietary problems; salty
tasting skin
10
--Tay-Sachs disease (chrom. 15)
highest incidence in Ashkenazic Jews
mental retardation, blindness,
deafness, and paralysis begin
around 6 months of age; death
normally occurs by 3-4 years
due to absence of an enzyme, Hex-A
(Hexosamindase-A), which allows a
lipid to accumulate in brain
--Sickle cell anemia (chrom. 11)
incidence highest in African/Americans
caused a change in a single amino
acid in hemoglobin, which prevents
some blood cells from carrying
oxygen, leading to a crescent shape
Oxygen depletion from exercise, plane
cabins, etc. leads to painful sickling of
blood cells. Heterozygotes show
protection from malaria.
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B. Sex-linked Inheritance
1. Y-linked Inheritance:
--Hairy ears
2. X-linked Inheritance: often recessive
genes on X-chromosome
Males have higher probability of phenotypic
expression than females.
--Red-green color blindness
--Hemophilia (detectable prenatally)
--Duchenne muscular dystrophy
--Lesch-Nylan disease
buildup of uric acid leads to
accumulation of salt crystals in
CNS, joints, kidneysrecurrent
vomiting, cerebral palsy, mental
retardation, self-mutilation & death
12
--Fragile X Syndrome: a genetic disorder
resulting in multiplication of part of
genetic code, resulting in a “pinched”
long leg on the X-chromosome
-on long arm of X, the triplet CAG
replicated 20-30 times; if replicated
over 50 times (up to 200), leads to
Fragile X-type symptoms (DeSalle
& Lindley, 1997)
incidence in 1 in 1000 births
most common genetic form of
mental retardation
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facial deformities (large ears,
prominent jaw, long narrow face), and
large testes in males
affected females may show reduced
intelligence, though most are normal
possible link to infantile autism
14
Food for thought:
Male fetuses are more likely than female
fetuses to be aborted, stillborn; infant and
childhood mortality is greater for males, as is the
rate of learning disabilities, behavioral disorders,
and mental retardation. This may be due to
genetic disorders (especially X-linked).
However, a greater number of boys are
conceived, and there are l06 male births for every
l00 females.
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III. CHROMOSOMAL ABNORMALITIES
Risk factors: Advanced Maternal Age
(sometimes advanced Paternal age)
--many caused by a glitch in meiosis
http://gslc.genetics.utah.edu/units/disorders/karyotype/
A. Autosomal Abnormalities
l. Down's Syndrome (Trisomy 2l)
--Incidence: 1/700 live births
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--Physical features: ipicanthal fold of eye,
flattened facial features, poor muscle
tone, short stature, and short broad hands
with an unusual crease in palms; excess
skin on back of neck
--Internal traits: congenital heart defects,
cataracts/visual impairments, deficiencies
in immune system leading to susceptibility
to infection and leukemia
--Shortened life expectancy; those who
live to over 35 typically develop the same
neuro-physiological and -psychological
symptoms as Alzheimer's patients
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--Cause: failure of chromosome 2l to
segregate during meiosis. Down's
syndrome children show a mosaicism-extra chromosomal material appears in
only some cells; degree of impairment
related to number of cells affected.
--Theories: Older Egg vs. “Relaxed
Selection” hypotheses.
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2. Edward’s Syndrome (Trisomy 18)
--Incidence: 1/5000-6000 live births;
predominantly females
--80-90% mortality rate by age 2
--Severe mental retardation; elfin
facial features (small nose & mouth,
receding chin, abnormal ears);
hearing loss; seizures; hypoglycemia
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20
3. Patau Syndrome (Trisomy 13)
--Incidence: 1/20,000 live births
--High mortality rate in 1st year
--Cleft lip & palate; congenital
heart defects; polydactyl; severe
mental retardation
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4. Cri-du-Chat (missing short arm of chr. 5)
--Incidence: 1 in 50,000 births;
Possibly normal life expectancy.
--Catlike cry; microencephaly;
congenital heart disease; severe
mental retardation; may be missing
kidney/s; sensitivity to loud noises;
low birth weight, partial webbing of
fingers or toes
--later, protuding teeth (normal sized
teeth in small head); curvature of spine;
developmental & language delays;
possibly self-mutilation & rocking
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B. Sex Chromosome Disorders
l. Turner Syndrome: X0
--Females in which one X-chromosome
(or part of it) is missing.
Incidence: l in l200-2500 females (90%
are spontaneously aborted).
--May not be detected until puberty,
when secondary sex characteristics
& menstruation do not develop.
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--Ovaries do not develop prenatally, the
girls maintain a very immature
appearance: Short stature (57 inches
average), a webbed neck. Increased
risk of fractures, strokes, diabetes &
cardiovascular problems.
--Hormone therapy may increase height
and induce menstruation; occasionally
pregnancy accomplished through in vitro
fertilization.
--Intelligence is often near average,
although with severe deficits in spatial
ability and directional sense (perhaps
due to smaller amounts of brain tissue—
grey & white--in parietal lobes; Reiss,
1995).
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2. Triple X Syndrome: XXX
Incidence: 1 in 500-1200 female (?)
--Normal sexual development
--delays in language development, lack of
motor coordination, poor academic
performance, and immature behavior.
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3. Klinefelter's Syndrome: XXY males
--Incidence: One in every 500-l000 births
--absence of development of male
secondary sex characteristics (facial
hair, deepening voice, increased muscle
structure) at puberty.
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--Underdeveloped testes & sterile (may
institute hormone therapy); Female-like fat
distribution, and potential breast
development at puberty
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--Tall, and tend to be overweight.
--Cognitive deficits: expressive language
delays in development, poor auditory
STM, reading difficulties, about 20% have
mild to moderate retardation
--self-esteem problems; prefer quieter
pursuits
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4. Jacob’s Syndrome: XYY
--Incidence: One in 2000 males.
--Above-average height, large teeth, and
sometimes severe acne
--Overrepresented in prison populations
(popular defense for a while), but not
necessarily more aggressive
[Representation in prisons possibly
due to decreased intelligence
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IV. METHODS OF PRENATAL TESTING
A. Non-invasive techniques
1. Genetic Counseling
--Information on a person's family tree is
gathered to ascertain the risk of certain
diseases.
--Genetic screening can be carried out on
parents to determine their genotype for
given disorders (e.g., Cystic Fibrosis, Tay
Sachs, sickle cell anemia).
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2. Triple Screen Test
(15th-16th medical weeks, up to 18th)
--Maternal blood test measures:
Alpha-feta protein (from fetal liver)
Estriol
hCG (human Chorionic Gonadotropin)
(inhibin A—increases detection of Down’s)
--high levels of Alpha-feta protein may
indicate neural tube defect (or baby is older
than thought, or (!) twins). 75-85% detection
--high levels of hCG, with low levels of estriol
& AFP, indicate elevated risk for Down’s
Syndrome. 60% detection in women < 35;
75% in women > 35
--low levels of all 3 may indicate higher
risk for trisomy 18 Edward’s syndrome)
--HIGH risk of “false positive” (46% in an
online poll of 23, 470 parents), and some
risk of false negatives
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3. Ultrasound (up to delivery)
--High frequency sound waves are
beamed into the uterus, and their
reflection reveals the size, shape, and
position of the fetus.
--a STRUCTURAL measure used to
monitor fetal growth, estimate gestational
age, detect multiple pregnancies,
depict placement of placenta, and detect
gross structural abnormalities
--can be done abdominally or
transvaginally
Figure 1 - Fetus with subcutaneous collection of fluid at the back of the neck
(often seen in Down’s Syndrome fetuses & those with other trisomies).
Image kindly provided by Dr Eva Pajkrt, University of Amsterdam.
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--growth abnormalities sometimes
indicative of genetic & chromosomal
abnormalities
--historically, has been claimed that
there are no known risks. However,
several studies in 1990s found:
-increase in left handedness in men,
indicating subtle brain damage (32%
increase with two pregnancy scans;
Swedish study)
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--cell damage due to :
-tissue heating (Uhlig, 1999)
-cavitation—sound waves may
cause bubbles to form & expand
--reduced fetal weight, fetal organ
weight, birth rate, decreased immune
system functioning, problems with
blood platelets, have been noticed in
humans & other species exposed to
ultrasound
http://educate-yourself.org/cn/2001/ ultrasoundandbraindamage19dec01.shtml
B. Invasive Techniques
*Usual analysis is for Chromosome
abnormalities; genetics analysis done only
if risk factors for a disease are present
--95% of fetuses examined are normal
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1. Chorionic Villa Sampling
(10-13 medical weeks)
--Chorion is the fetal membrane that will
form the fetal side of the placenta
--a tissue sample is removed from
chorion by pushing narrow needle in
15-20 times
--Complications:
-1-2% risk of inducing miscarriage
-may interfere with vascularization
--> limb deformities
-Mosaicism a problem for females
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2. Amniocentesis
(14-16th medical weeks)
--Fetal cells are extracted from the
amniotic fluid and cultured for
chromosome/genetic analysis
--Complications:
-Risk is .33-.5% (1/300 to 1/200) for
inducing miscarriage [hence
benchmark of age 35, where
risk of any chromosome
abnormality is 1/180]
-Risk is even higher with early
amniocentesis (11-14th weeks)
-Rh- mothers need Rhogam
because of risk of co-mixing
blood of mother & baby
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3. Fetoscopy
(18-22 medical weeks, or after 16th)
--miniature telescope-like instrument w/
light & lenses inserted into tiny incision
into amniotic sac
--can detect structural deformities
--Blood samples taken from junction of
umbilical cord/placenta, and/or tiny bit of
fetal or placental tissue removed
--Complications:
-up to 12% risk of inducing
miscarriage
-up to 47% risk of rupturing
membranes, necessitating
pre-term delivery
-Rh- mothers need Rhogam
risk of co-mixing
blood of mother & baby
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V. HEREDITY INFLUENCES on
BEHAVIOR
A. Estimating Hereditability
1. Hereditability refers to the extent
that individual differences within some
population are due to genetics
(e.g., hereditability for having 2 eyes is
0.00—having two eyes is clearly based on
genes, but no individual differences)
2. Concordance Rates & Correlations
--Determine hereditability by measuring a
trait in individuals with shared genes and/or
shared environments:





Biological parents, siblings
Identical twins, raised together or apart
Fraternal twins, raised together or apart
Adoptive parents & adopted child
Biological parents & adopted child
3. What does the Correlation mean?
Correlation of r = 0.70; if squared,
provides amount of variance due to genetics =
.49 (or 49%)
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4.
Many
personality
traits
(e.g.,
Intro/Extroversion & Empathy) and Intellectual
Traits/skills clearly have an inherited component.
Average Correlations between IQ scores
(Bouchard & McGue, 1981; *Pederson et al, 1985; **Segal, 2000)
Family Pairs
Identical Twins
Raised
Together
.86
Raised
Apart
.72
Fraternal Twins
.60
.52*
Biological Siblings
.47
.24
Biological Parent & Child
.42
.22
Half Siblings
.31
--
Adopted Siblings
.34
--
Adoptive Parent & Adopted Child
.19
--
Unrelated siblings (same age, same home)
.26**
--
Hereditability of Temperament: Correlations of Angry Emotionality (Plomin
et al., 1988)
Raised Together
Raised Apart
Identical Twins
.37
.33
Fraternal Twins
.17
.09
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Hereditability of Certain Traits from Minnesota Twin Study
(Bouchard et al., 1990)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Heig.
Weig.
S ys. BP
IQ
Person.
Relig.
S oc. Att.
Height
Syst. BP
Personality
Social Attitudes
Weight
IQ
Religiosity
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5. However, environment also impacts
almost all inherited skills or traits.
Gene Environment Interactions: Odds of Depression at age 26
(Caspi et al., 2003)
0.45
0.4
0.35
0.3
2 hi-risk genes
Heterozygous
2 protective genes
0.25
0.2
0.15
0.1
0.05
0
0
1
2
3
4
Number of Stressful Childhood Events
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B. Interaction of Genes & Environment
l. Canalization (Conrad Waddington): the
degree to which a trait is constrained by
genetics. Some traits (e.g., crawling) more
canalized than others (e.g., playing soccer).
Think of a river (a child's development)
seeking its course over terrain. Deeply
canalized sections will mold the river;
shallower sections will be molded BY the
river.
Some traits may be heavily canalized early in
development (e.g., language development),
but less so later on (e.g., reading ability).
2. Range-of-Reaction (Gottesman, 1963):
Genes set boundaries & establish a range
of reactions; because of different genotypes,
individuals will respond differently to the
same environment
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3. Niche-picking (Scarr & McCartney, 1983):
Genotype contributes propensities toward
certain skills and abilities, and we then seek
activities which are compatible with our
genetic endowment.
(Active Genotype/Environment interaction)
4.
Correlations between genetics and
environment can also occur:
a. Passively, as when parents set up an
environment consistent with their (and their
child's) predispositions (e.g., sociability); or
b. Evocatively, as when a child’s traits
influence the behavior of those around
him/her (e.g., if a shy child is less engaged)
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