Chapter 2
The Start of Life: Genetics and Prenatal Development
Chapter
Outline
I.
Heredity
A. Genes and Chromosomes: The Code of Life
1. Humans begin life as a
single cell.
2.
Our genetic code is stored and communicated in our GENES, the
basic units of genetic information.
a)
Genes are composed of sequences of DNA
(deoxyribonucleic acid) the substance that determine
the nature of every cell in the body and how it will function.
b) Humans have over 30,000
genes.
c)
Genes are arranged in specific locations and in a specific order along
46 CHROMOSOMES, rod-shaped portions of DNA that are organized in 23 pairs.
(1) One pair of chromosomes (via the gametes) is provided by the mother; one by the father at fertilization.
d)
GAMETES (sperm and ova) are formed by a process called meiosis.
e)
All other cells replicate the genetic code by a process called mitosis.
f) There are tens of trillions
of possible genetic combinations.
3. Multiple births
a) Less than 2 percent of all
pregnancies produce twins, the odds are slimmer for three or more children.
b)
MONOZYGOTIC TWINS, who are genetically identical,
form when a cluster of cells in the ovum splits off within the first two weeks
following fertilization.
c)
DIZYGOTIC TWINS, who are produced when two separate ova are fertilized by two separate
sperm, are no more genetically
similar than two siblings.
d) Other kinds of multiple
births (triplets, quadruplets, etc.) can form from either mechanism.
e) Using fertility drugs
increases the chances of having a multiple birth.
(1) 1 in 10 are dizygotic
(2) Older women are more likely
to have multiple births.
f) Racial and ethnic
differences affect the rate of multiple births.
(1) Caucasian - 1 out of 86 dizygotic
(2) African American - 1 out of
70 dizygotic
4. The 23rd chromosome
determines the sex of the child.
a) Females are XX.
b) Males are XY.
c) The father's sperm
determines the sex of the child.
B. The Basics of Genetics
1. An Austrian monk, Gregor Mendel (mid 1800s), working with peas, discovered
that when two competing traits were present only one could be expressed.
a) The trait that is expressed when two competing traits are present is
called the DOMINANT TRAIT.
b) The trait that is present in the organism but not expressed is called
the RECESSIVE TRAIT.
c) GENOTYPE is the underlying combination of
genetic material present (but not outwardly visible) in an organism.
d) PHENOTYPE is an observable trait, the trait that is actually seen.
e) Alleles are genes for traits
that may take alternate forms.
(1) HOMOZYGOUS is inheriting from patents
similar genes for a given trait.
(2) HETEROZYGOUS is inheriting from parents
different forms of a gene for a given trait.
f) If a child receives a recessive allele from each parent, it
will display the recessive characteristic.
g) Most traits are the result of POLYGENIC INHERITANCE,
in which a combination of multiple gene
pairs is responsible for the production of a particular trait.
h) Some genes (such as those
for blood type AB) are neither dominant nor recessive but are a combination.
i)
Some recessive genes are
X-LINKED GENES, meaning they are
located on the X chromosome.
(1) Males have a higher risk for
a variety of X-linked disorders because they lack a second X chromosome to
counteract the genetic information that produces the disorder.
(2) Hemophilia is a blood disorder
produced by X-linked genes.
(3) red-green color blindness is another.
2.
In 2001, molecular biologists succeeded in mapping the human genome –
the specific sequence of genes on each chromosome.
a)
The number of human genes has been revised downward from 100,000 to
30,000.
b) Humans share 99.9 percent of
the gene sequence.
3. The most recent approach to the study of the effects of heredity on
behavior and development is called BEHAVIORAL
GENETICS.
a)
This field merges psychology – the study of behavior – with genetics –
the study of transmission of characteristics through heredity.
b) These researchers are
learning how behavioral difficulties (such as schizophrenia) may have a genetic
basis.
c) Researchers also seek to
identify how genetic defects may be remedied.
C. Inherited and Genetic
Disorders: When Development Goes Awry
1. Some genetic disorders are
inherited (e.g., PKU).
2.
Some genetic disorders are the result of genes that become physically
damaged.
a) Sometimes genes
spontaneously change their form, a process called spontaneous mutation.
b) Certain environmental
factors, such as exposure to X-rays, can produce malformed genetic material.
c) Some genetic disorders
include
(1) DOWN SYNDROME is a disorder produced by the
presence of an extra chromosome on the 21st chromosome pair, once referred to
as mongolism.
(2) SICKLE-CELL ANEMIA is a blood disorder that gets
its name from the shape of the red blood cells in those who have it.
(3) FRAGILE X SYNDROME is a disorder produced by injury to a gene
on the X chromosome, producing mild to moderate mental retardation.
(4) TAY-SACHS DISEASE is an untreatable disorder that
produces blindness and muscle degeneration prior to death.
(5) One male out of every 400 is
born with KLINEFELTER'S SYNDROME, a disorder resulting from the presence of an
extra X chromosome that produces underdeveloped genitals, extreme height, and
enlarged breasts.
II. GENETIC COUNSELING is the discipline that focuses on helping people deal with issues
related to inherited disorders.
A.
Genetic counselors use a variety of data.
1.
They can take a thorough family history, seeking any familial incidence
of birth defects.
2.
The age of mother and father will be taken in account.
3. Blood, skin, and urine may
be used to isolate and examine specific chromosomes.
4. Possible genetic defects can
be identified by assembling a karyotype, a chart containing enlarged photos of each of the
chromosomes.
5. Other tests take place once
the woman is already pregnant:
a) AMNIOCENTESIS is 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.
b) CHORIONIC VILLUS SAMPLING (CVS) is a test
used to find genetic defects that involves taking samples of hairlike material that surrounds the embryo.
c) ULTRASOUND SONOGRAPHY is a process in which
high-frequency sound waves scan the mother's womb to produce an image of the
unborn baby whose size and shape can then be assessed.
B.
Huntington’s disease, and about 450 other
disorders can be predicted on the basis of genetic testing.
III. The Interaction of Heredity
and Environment
A. The role of the Environment in Determining the Expression of Genes: From Genotypes to Phenotypes
1. An individual's TEMPERAMENT, the patterns of arousal and emotionality that represent consistent and
enduring characteristics, may represent MULTIFACTORIAL TRANSMISSION, traits
that are determined by a combination of both genetic and environmental factors
in which a genotype provides a range within which a phenotype may be expressed.
2. Some genotypes are not as
sensitive to the environment as others.
B. Answering the Nature-Nurture
Riddle
1.
The correct question is not whether behavior is caused by nature or
nurture but how much by nature and how much by nurture.
2. Nonhuman studies
a)
Scientists put laboratory animals bred to share genetic backgrounds in
different environments to explore the effects of these environments.
3. Conversely, they use genetically
different animals in similar environments to determine the role of genetics.
C. Human studies
1. Scientists use human twins
to study the effects of genes and the environment.
2. Differences between
monozygotic twins separated at birth are most likely but not always due to
different environments.
3. If monozygotic twins are
more similar than dizygotic twins on a particular
trait then we can assume that genetics plays a role.
4. People who are unrelated but
share the same environment also tell us about environmental influences.
5. Researchers also study
biological parents and their children versus adoptive parents and their
children to see the effects of heredity versus environment.
6. Bottom line: Virtually all traits, characteristics, and
behaviors are the joint result of the combination and interaction of nature and
nurture.
D. The more genetically similar
two people are, the more likely they are to share physical characteristics
(e.g., height, weight).
E. Genetics plays a significant
role in intelligence; however, the environment is also a significant factor.
F. Increasing evidence supports
the conclusion that at least some personality traits have at least some genetic
components.
1. neuroticism refers to the degree of moodiness,
touchiness, or sensitivity an individual characteristically displays.
2. extroversion is the degree to which a
person seeks to be with others, to behave in an outgoing manner, and generally
to be sociable.
3. Certain traits reflect the
contribution of genetics more than others.
a) social potency
b) traditionalism
4. political attitudes, religious
interests and values, even attitudes toward human sexuality seem to have
genetic components.
G. Developmental Diversity: Might a Culture’s Philosophical Outlook Be
Determined by Genetics?
1.
Jerome Kagan and colleagues speculate that
the underlying temperament of a given society, determined by genetics, may
predispose people in that society toward a particular philosophy.
H. Several psychological
disorders have been shown to be related, at least in part, to genetic factors:
1. schizophrenia
2. major depression
3. alcoholism
4. autism
5. attention-deficit
hyperactivity disorder
I.
According to Sandra Scarr, the genetic
endowment provided by parents to children can determine genetic characteristics
and influence their environment.
1. With ACTIVE GENOTYPE-ENVIRONMENT EFFECTS, children focus on those aspects of the environment that are most
congruent with their genetically determined abilities.
2. With PASSIVE GENOTYPE-ENVIRONMENT EFFECTS, a parent's genes can produce an environment that influences the child.
3. With EVOCATIVE GENOTYPE-ENVIRONMENT EFFECTS, the child's genes elicit a particular type of environment.
IV.
Prenatal Growth and Change
A. Fertilization: The Moment of Conception
1. FERTILIZATION is the process by which
a sperm and an ovum – the male and female gametes – join to form a single new
cell.
B. Alternate Routes to
Pregnancy: Giving Nature a Boost
1. 15 percent of couples suffer from INFERTILITY, the inability to conceive after 12 to 18 months of trying to become
pregnant.
2. Infertility is produced by
several causes:
a) the age of the parents
b) previous use of birth
control pills, illicit drugs or cigarettes, STDs
c) men who have an abnormally
low sperm count
d) the woman's mother taking certain drugs during
pregnancy
3. There are several approaches
to conception.
a) ARTIFICIAL INSEMINATION is a process
of fertilization in which a man's sperm is placed directly into a woman's
vagina by a physician.
b) IN VITRO FERTILIZATION (IVF) is 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.
c)
SURROGATE MOTHER, a woman who agrees to carry the child to term, may be used in cases
where the mother is unable to conceive.
4. Evidence suggests that the
quality of parenting in families who have used artificial means to conception
may even be superior to naturally conceived children.
5. The psychological adjustment of children conceived artificially is no different than that of children conceived using natural techniques.
C. The Stages of the Prenatal
Period: The Onset of Development
1. The prenatal period consists
of three phases.
a)
The GERMINAL STAGE is the
first and shortest stage of prenatal development, which takes place during the
first two weeks following conception.
(1)
It is characterized by methodical cell division and the attachment of
the organism (blastocyst)
to the wall of the uterus.
(2)
The baby is called a zygote
at this stage.
(3)
The cells become specialized with some forming a protective layer
around the zygote, while others create:
(a)
PLACENTA the conduit between the
mother and fetus,
providing nourishment and oxygen via the umbilical cord.
b)
The second stage is called the EMBRYONIC
STAGE, the period from two to eight weeks following fertilization during
which significant growth occurs in the major organs and body systems.
(1) At this point the child is
called an embryo.
(2) The developing child is now
composed of three layers:
(3) the ectoderm is the outer layer forming the skin, hair, teeth, sense
organs, the brain and spinal cord;
(4) the endoderm is the inner layer producing the digestive system, liver,
pancreas, and respiratory system;
(5) the mesoderm is sandwiched between the inner and outer layers and forms
the muscles, bones, blood, and circulatory system.
c)
The FETAL STAGE begins about
eight weeks after conception and continues until birth.
(1)
The developing child from eight weeks after conception until birth is
called a FETUS.
(2)
The fetus dramatically increases in size and weight.
(3)
Organs become more differentiated and operational.
(4)
By three months the fetus swallows and urinates.
(5)
By four months the mother will be able to feel her fetus move.
D. Miscarriage and Abortion
1.
A miscarriage – known as spontaneous abortion – occurs when
pregnancy ends before the developing child is able to survive outside the womb.
a) 15 to 20 percent of all
pregnancies end in miscarriage.
b) Many times the mother is not
even aware she is pregnant.
c)
Typically, miscarriages are attributable to some sort of genetic
abnormality.
E. The Prenatal
Environment: Threats to Prenatal
Development
1. Certain aspects of mothers'
and fathers' behavior, both before and after conception, can produce lifelong
consequences for the child.
a) Some of the most profound
consequences are brought about by a TERATOGEN,
an environmental agent such as a virus,
chemical, or other factor that produces a birth defect.
b) At some phases of prenatal
development, a teratogen may have minimal impact; at
other periods, consequences can be severe.
(1)
A mother's diet clearly plays an important role in bolstering the
development of the fetus.
(2) Research shows that mothers
over 30 and adolescent mothers are at greater risk for a variety of pregnancy
and birth complications:
(a) premature birth
(b) low birth weight
(c) Down syndrome
(d) higher infant mortality
rates
(3) Illness in a pregnant woman
can have devastating consequences:
(a) Rubella (German measles) prior to the 11th week can cause blindness, deafness,
heart defects, or brain damage.
(b) Chicken pox and mumps may cause birth
defects and miscarriage, respectively.
(c) Syphilis and gonorrhea can be
transmitted to the child.
(d) Babies born with AIDS (acquired
immune deficiency syndrome) can have birth abnormalities including small,
misshapen faces, protruding lips, and brain deterioration.
(i)
30 percent of infants born to mothers with AIDS have the virus.
(ii)
90 percent have neurological
delays and deficits in motor coordination, speech, and facial expression.
(iii)
In addition, they are
susceptible to infection.
(iv) Survival past infancy is rare.
(4) Mother's use of legal and
illegal drugs pose serious risks to the unborn child:
(a) aspirin can lead to
bleeding;
(b) thalidomide caused missing limbs;
(c) DES (diethylstilbestrol) later caused cervical and vaginal cancer in
daughters;
(d) marijuana restricts oxygen to the fetus;
(e) 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.
(5) Both alcohol and cigarettes
can disrupt the development of the fetus:
(a) 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.
(b) Even
smaller amounts of alcohol can produce FETAL ALCOHOL
EFFECTS (FAE), a condition in which children display some,
although not all, of the problems of FAS due to their mother’s consumption of
alcohol during pregnancy.
(c) just two drinks a day has been
associated with lower intelligence.
(d) smoking reduces the oxygen content
and increase carbon monoxide.
(i)
babies
can miscarry or are born with abnormally low birth weight.
(ii)
babies
born to smokers are shorter and may be 50 percent more likely to have mental
retardation.
(e) Fathers can affect the
prenatal environment:
(i) Secondhand smoke can affect
the mother’s health.
(ii) Alcohol and illegal drugs
can lead to chromosomal damage at conception.
(iii) Stress may produce an
unhealthy environment for the mother.
Key Terms and Concepts
Genes
DNA (deoxyribonucleic acid)
Chromosomes
Zygote
Gametes
Monozygotic twins
Dizygotic
twins
Dominant trait
Recessive trait
Genotype
Phenotype
Homozygous
Heterozygous
Polygenic inheritance
X-linked genes
Behavioral genetics
Down syndrome
Fragile X syndrome
Sickle-cell anemia
Tay-Sachs disease
Klinefelter’s
syndrome
Genetic counseling
Amniocentesis
Chorionic villus sampling
Ultrasound sonography
Temperament
Multifactorial
transmission
Active genotype-environment effects
Passive genotype-environment effects
Evocative genotype-environment effects
Fertilization
Infertility
Artificial insemination
In vitro fertilization
Surrogate mother
Germinal stage
Placenta
Embryonic stage
Fetal stage
Fetus
Teratogen
Fetal alcohol syndrome (FAS)
Fetal alcohol effects (FAE)