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



DNA (deoxyribonucleic acid)




Monozygotic twins

Dizygotic twins

Dominant trait

Recessive trait





Polygenic inheritance

X-linked genes

Behavioral genetics

Down syndrome

Fragile X syndrome

Sickle-cell anemia

Tay-Sachs disease

Klinefelter’s syndrome

Genetic counseling


Chorionic villus sampling

Ultrasound sonography


Multifactorial transmission

Active genotype-environment effects

Passive genotype-environment effects

Evocative genotype-environment effects



Artificial insemination

In vitro fertilization

Surrogate mother

Germinal stage


Embryonic stage

Fetal stage



Fetal alcohol syndrome (FAS)

Fetal alcohol effects (FAE)