Chapter 2
The Start of Life: Genetics &
Prenatal Development
What is Heredity?
• Inheritance! The transmission of
qualities genetically derived from
one’s ancestors
• As humans, we begin life as a single
cell!
(1/20,000,000th of an ounce!)
This single cell is transformed
into something resembling a
person by the human genetic
code!
• How is the human genetic code
transferred??
– GAMETES
• The sex cells from the mother and father that
form a new cell at conception (also known as
sperm and ovum)
• Fertilization is the process by which a sperm and
an ovum (the gametes) join go form a single new
cell, called a zygote
– Some important things to remember about
fertilization:
• Females are born with all their ova (about
400, 000!; the ova mature only when the
female reaches puberty; an ovum ripens
every 28 days)
• Males produce several hundred million
sperm daily!
• Fertilization typically takes place in the
fallopian tubes
Genes & Chromosomes: The
Code of Life
• Genes are the basic unit of genetic
information
– Composed of DNA (deoxyribonucleic
acid) sequences
• DNA determines the nature of every
cell in the body and how it will
function
(More about genes)
• Humans have over 100, 000 genes!
• They are arranged in specific locations and
in a specific order along 46 chromosomes
– (chromosomes are rod-shaped sections of DNA
organized into 23 pairs)
Some more things to know about chromosomes…
• One pair of chromosomes is provided (through the
gametes) by the mother
• One pair of chromosomes is provided by the father at
fertilization
– Gametes (sperm & ova) are formed by a process
called meiosis
– ALL other cells replicate the genetic code by a
process called mitosis
• THERE ARE 10’S OF TRILLIONS OF
POSSIBLE GENETIC COMBOS!
Some important things to know
about multiple births…
• Less than 2% of ALL pregnancies
produce twins!! (the odds are even
slimmer for triplets, quadruplets, etc.!)
A Comparison of Twins…
• MONOZYGOTIC
Twins
– Genetically
identical; form
when cluster of
cells in the ovum
splits off within the
first 2 weeks
following
fertilization
• DIZYGOTIC Twins
– 2 separate ova are
fertilized by 2
separate sperm; no
more genetically
similar than 2
siblings
Other kinds of multiple births…
• Triplets, quadruplets, etc. can be either monozygotic or
dizygotic
• The use of fertility drugs increases the chance of having
multiple births]
– 1 in 10 that the birth will be dizygotic
– Twin births are up 42 % under these circumstances!
• Racial & Ethnic differences affect the rate of multiple births
– African American: 1 out of 70 births are dizygotic
– Caucasians: 1 out of 86 dizygotic
– Chinese Americans 1 out of 300 dizgotic
Some important facts:
The 23rd chromosome determines the sex of
the child!!
* Females are XX
* Males are XY
* The FATHER’s sperm determines the
sex of the child
The Basics of Genetics…
• Gregor Mendel
– Worked with peas; discovered that when 2
competing traits were present, only one could
be expressed
• The DOMINANT TRAIT is the trait that is
expressed
• The RECESSIVE TRAIT is present in the
organism but not expressed
(More basics of genetics…)
• A GENOTYPE is the underlying combo of genetic
material present (but not outwardly visible) in an
organism
• A PHENOTYPE is an observable trait that is
actually seen; physical appearance
-Alleles are genes for traits that may take alternate
forms (example: hair texture/color, skin tone)
– Homozygous (inheriting similar genes for a
given trait )
– Heterozygous (inheriting different forms of a
gene for a given trait)
[Alleles (genes for traits that make take alternate forms)
continued]
• A recessive allele from each parent the
child displays the recessive characteristic
• A dominant allele received from each parent
 a dominant characteristic is displayed
* MOST traits are the result of POLYGENIC
INHERITANCE (a combo of multiple gene
pairs leads to a particular trait)
• Some genes are neither dominant nor recessive,
but instead are a combination
– AB blood type
• Some are x-linked genes
– Located on the x chromosome
Males have higher risk for x-linked disorders
Why?! Because the lack a 2nd X chromosome
to counteract the genetic info that produces
the disorder!
examples of x-linked disorders: redgreen color blindness, hemophilia
PKU is an inherited disorder
in which a child is unable to
use an essential amino acid,
and allows a build up of
toxins causing brain
damage and mental
retardation. We can see the
transmission of genetic
information in humans by
considering the
transmission of this
disorder.
-neither parent carries gene: child cannot
develop PKU
-one parent carries: child cannot develop
-both parents: child has 1 in 4 chance
The MOST recent approach to
the study of the effects of
heredity on behavior &
development:
Behavioral Genetics
The Human Genome Project…
~In early 2001 molecular biologists succeeded in
mapping the sequence of genes on chromosomes
-one of the most important moments in the history
of genetics!
-already leading to important advances in our
understanding of genetics
-99.9% of gene sequence is shared by all
humans: similarities of people realized!
- # of human genes less than thought
(30,000 rather than 100,000+)
- Will also help id disorders
Uniquely Human?
The human genome
project has lead to the
realization that humans
are less complex than
originally thought, and
not much different that
some primitive species!
Behavioral geneticists investigate several
areas…
• Behavioral geneticists explore how
behavioral difficulties may have a
genetic basis (example: schizophrenia)
• How genetic defects may be remedied
• The inheritance patterns of genetic
disorders
(the focus of behavioral geneticists continued…)
• How physically damaged genes
contribute to genetic disorders
• The role of spontaneous mutation in
genetic disorders (how genes
sometimes change form on their own)
• How environmental factors affect
genetic mutation (x-ray exposure,
teratogens)
Some genetic disorders include…
• Down Syndrome
– A disorder produced by the presence of an extra
chromosome on the 21st chromosome pair
• Sickle-Cell Anemia
– A blood disorder (named for the shape of the
disordered blood cells)
(More genetic disorders include…)
•
Tay-Sachs Disease
–
•
An untreatable disorder; produces blindness,
muscle degeneration prior to death
Klinefelter’s Syndrome
-- Results from the presence of an extra X
chromosome that produces underdeveloped
genitals, extreme height, and enlarged breasts.
Inheriting Hemophilia…
Hemophilia is a blood-clotting disorder that has been an inherited
problem in the royal families of Europe, as illustrated above in the
descendants of Queen Victoria of Britain.
What is Genetic Counseling??
•
The discipline that focuses on helping
people deal with issues related to inherited
disorders
–
Blood, skin, urine often used to
isolate/examine specific chromosomes
-- Possible genetic defects can be identified by
assembling a karyotype, a chart containing
enlarged photos of each of the chromosomes.
Other tests that can take place once a woman is pregnant:
AMNIOCENTESIS (the process of identifying genetic
defects by examining a small sample of fetal cells
drawn by a needle inserted into the amniotic fluid
surrounding the unborn fetus).
CHORIONIC VILLUS SAMPLING (CVS) (a test used to
find genetic defects that involves taking samples of
hairlike material that surrounds the embryo).
ULTRASOUND SONOGRAPHY (a process in which highfrequency sound waves scan the mother's womb to
produce an image of the unborn baby whose size and
shape can then be assessed).
Some Facts About Infertility…
•
About 15 % of couples suffer from infertility
(inability to conceive after 12 to 18 months of
trying)
• There are several causes:
– Parental age
– Previous use of birth control pills, illicit drugs
or cigarettes, STDs
– men who have an abnormally low sperm
count
– the woman's mother taking certain drugs
during pregnancy
Several Alternate Routes to Pregnancy…
--
--
--
ARTIFICIAL INSEMINATION (fertilization
that occurs after a man's sperm is placed
directly into a woman's vagina by a physician).
IN VITRO FERTILIZATION (IVF) (a
procedure 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).
SURROGATE MOTHER, a woman who
agrees to carry the child to term, may be used in
cases where the mother is unable to conceive.
 Evidence suggests that the quality of
parenting in families who have used
artificial means to conception may
even be superior to naturally
conceived children.
 The psychological adjustment of
children conceived artificially is no
different than that of children
conceived using natural techniques
Nature vs. Nurture…
The interaction of heredity vs
environment
~The correct question is not whether
behavior is caused by nature or
nurture but how much by nature
and how much by nurture
Possible Causes of Intelligence: Nature vs. Nurture
Intelligence may be explained by a range of
possible causes, spanning the nature-nurture
continuum. Which explanation do you find most
convincing, based on information in this chapter?
How do scientists investigate the effects of
heredity & environment on development
anyway??
A. Nonhumans studies
- Laboratory animals
* bred to share genetic backgrounds and
placed in different environments to explore
the effects of these environments.
* conversely, they use genetically different
animals in similar environments to
determine the role
(How scientists investigate the effects of heredity &
environment, continued)
•
Human studies
 Human twins used to study the effects of
genes and the environment.
 Differences between monozygotic twins
separated at birth (usually most likely but
not always due to different environments).
 If monozygotic twins are more similar than
dizygotic twins on a particular trait than we
can assume that genetics plays a role.
(Human studies in heredity/environment research,
continued)
 People who are unrelated but share the
same environment also tell us about
environmental influences.
 Researchers also study biological parents
and their children versus adoptive
parents and their children to see the
effects of heredity versus environment.
 Bottom line: Virtually all traits,
characteristics, and behaviors are the
joint result of the combination and
interaction of nature and nurture.
No issue looking at the influence of heredity and
environment has more research than the topic of
intelligence, because it is a core human characteristic!
The closer the genetic link between two individuals, the
more similar their IQ scores are…
More about the interaction of heredity &
environment in effecting development…
--The more genetically similar two people are, the
more likely they are to share physical
characteristics (e.g., height, weight).
--Genetics plays a significant role in intelligence;
however, the environment is also a significant
factor.
--Increasing evidence supports the conclusion
that at least some personality traits have at
least some genetic components. Some
personality characteristics have been found to
be linked to genetic factors (neuroticism,
extroversion)
Genetic & Environmental Influences On
Personality
• Increasing evidence supports the conclusion
that at least some personality characteristics
are affected by genetic factors
– Neuroticism (emotional reactivity)
– Extroversion (outgoing/sociability)
~How do we know which personality traits reflect
genetics?
– Examination of genes
– Twin studies
Inheriting personality traits…
These
personality
traits have
been found
to have
genetic
components.
The higher
the
percentage,
the greater
the
influence of
heredity.
Some Psychological Disorders at Least
Partially Related to Genetic Factors…
A.
B.
C.
D.
E.
schizophrenia
major depression
alcoholism
autism
attention-deficit hyperactivity disorder
The
psychological
disorder of
schizophrenia
has clear
genetic
components.
The closer the
genetic link,
the more
likely a
relative is to
develop
schizophrenia.
Psychological Disorders:
Predisposition or Automatic
Inheritance?
• Genetics alone does not automatically lead to the
development of schizophrenia or other
psychological disorders.
– If genetics were the sole cause, identical twins
would automatically develop schizophrenia,
and this is not the case!
• Other factors contribute, including
structural abnormalities, biochemical
imbalances, and stressors
Can Genes Influence Environments?
• Developmental psychologist Sandra Scarr says yes—
genetics can influence environment!
– 3 ways…
1. Active genotype-environment effects (kids focus
on aspects of the environment that are congruent
with their genetic abilities)
– Noticing the music tryouts flyer over the sports
tryout flyer
(Sandra Scarr, genetics & environment continued…)
2. Passive genotype-environment effects (parents
genes are associated with environment where
kids are raised)
– Parents are sports oriented= more
opportunities for kids in this area
3. Evocative genotype-environment effects (kids
genes elicit a type of environment
– Kid with athletic ability draws attention to this
so parents will nurture
The Start of Life…
Genetics & Prenatal Developmental Development
• Fertilization: The Moment of Conception
– The joining of the sperm and ovum to create the
single-celled zygote that develops into a baby
• Females are born with 400,000+ ova, which
mature at puberty
• Males produce sperm with a much shorter
lifespan! Several hundred million sperm per
day in adult males!
Anatomy of the Sex Organs…
Routes to Pregnancy…
~Sexual Activity
• During sexual activity, the sperm released wind
their way to the ovum. One sperm fertilizes an
ovum, and together contain all the genetic data
to produce a new human.
• Artificial insemination (sperm placed in vagina
by physician)
~In vitro fertilization (ova removed and
fertilized by sperm in a lab)
~Surrogate Mother (another woman carries a
couples baby)
The Stages of the Prenatal Period:
The Onset of Development
Prenatal Growth & Change
• Developmentalists divide the
prenatal period into three phases:
the germinal, embryonic, and fetal
stages.
The 3 Phases of the Prenatal
Period:
1) GERMINAL STAGE
- the first and shortest stage of prenatal
development
- takes place during the first two weeks
following conception.
- characterized by methodical cell division
and the attachment of the organism
(blastocyst) to the wall of the uterus.
- The baby is called a zygote at this stage.
(The 3 Phases of the Prenatal Period, continued)
2)
The EMBRYONIC STAGE
-
the period from two to eight weeks following fertilization
during which significant growth occurs in the major
organs and body systems.
At this point the child is called an embryo.
The developing child is now composed of three layers:
- the ectoderm (the outer layer forming the skin, hair,
teeth, sense organs, the brain and spinal cord)
- the endoderm (the inner layer producing the digestive
system, liver, pancreas, and respiratory system)
- the mesoderm (sandwiched between the inner and
outer layers and forms the muscles, bones, blood,
and circulatory system).
(The 3 Phases of the Prenatal Period, continued)
3) The FETAL STAGE begins about eight
weeks after conception and continues until
birth.
~ The developing child from eight weeks after
conception until birth is called a FETUS.
~ The fetus dramatically increases in size and
weight.
~ Organs become more differentiated and
operational.
~ By three months the fetus swallows and
urinates.
~ By four months the mother will be able to feel
her fetus move
During the fetal
period, the
proportions of the
body change
dramatically!
Threats to Prenatal Development…
Certain aspects of mothers' and fathers'
behavior, both before and after conception,
can produce lifelong consequences for the
child.
 TERATOGENs (environmental agents such
as a virus, chemical, or other factor that
produces a birth defect. )
-- At some phases of prenatal development,
a teratogen may have minimal impact; at
other periods, consequences can be
severe.
Teratogen Sensitivity Various parts of the body are
more sensitive to teratogens at
different stages of development
Other threats to prenatal development…
-- A mother's diet clearly plays an important role
in bolstering the development of the fetus.
-- Research shows that mothers over 30 and
adolescent mothers are at greater risk for a
variety of pregnancy and birth
complications:
• premature birth
• low birth weight
• Down syndrome
• higher infant mortality rates
• Illness
Other threats to prenatal development…
 Illness in a pregnant woman can have devastating
consequences:
- Rubella (German measles) prior to the 11th week can
cause blindness, deafness, heart defects, or brain
damage.
- Chicken pox and mumps may cause birth defects and
miscarriage, respectively.
- Syphilis and gonorrhea can be transmitted to the child.
- Babies born with AIDS can have birth abnormalities
including small, misshapen faces, protruding lips, and brain
deterioration. 90 percent have neurological delays and
deficits in motor coordination, speech, and facial
expression. In addition, they are susceptible to infection.
Survival past infancy is rare.
More threats to prenatal development
 Mother's use of legal and illegal drugs pose
serious risks to the unborn child:
- aspirin can lead to bleeding
-thalidomide caused missing limbs
-marijuana restricts oxygen to the fetus
-cocaine restricts blood flow and oxygen, babies
are born addicted and go through withdrawal;
they are shorter and weigh less; they have
serious respiratory problems and birth defects or
seizures; it is often impossible to soothe them.
Still more threats to prenatal development
Both alcohol and cigarettes can disrupt the
development of the fetus:
-just two drinks a day has been
associated with lower intelligence;
~~FETAL ALCOHOL SYNDROME (FAS)
is a disorder caused by the pregnant
mother consuming substantial quantities
of alcohol during pregnancy potentially
resulting in mental retardation, delayed
growth, and facial deformities;
(threats to prenatal development)
--smoking reduces the oxygen content and
increase carbon monoxide; babies can
miscarry or are born with abnormally low birth
weight; babies born to smokers are shorter and
may be intellectually delayed.
Fathers can affect the prenatal environment
(second hand smoke and the environment)
•Don’t forget to read chapter
3 for next time!!!!!