CHAPTER 2
Heredity and Prenatal Development
The Influence of Heredity
on Development
Chromosomes and Genes
• Genetics - the field of biology that studies heredity
– Fundamental in the transmission of physical traits
– Also plays a role in psychological traits
• Chromosomes – Rod-shaped structures composed of
genes that are found within the nuclei of cells
• Gene – The basic unit of heredity. Composed of DNA
• Polygenic – Resulting from many genes
• Deoxyribonucleic Acid (DNA) – Genetic material that
takes the form of a double helix composed of
phosphates, sugars and bases.
Identical and Fraternal Twins
• Monozygotic (MZ)
- Zygote divides into two cells that separate so that each
develops into an individual with the same genetic makeup.
• Dizygotic (DZ)
- Two ova are produced in the same month, each
fertilized by a different sperm cell.
• Ovulation
- As women reach end of childbearing years, ovulation
becomes less regular.
-Results in months when more than one ovum is released
Chromosomal and Genetic Abnormalities
• Down syndrome
– Caused by an extra chromosome of the 21st pair,
resulting in 47 chromosomes
• Characteristic features
-rounded face
-protruding tongue
-broad, flat nose
-sloping fold of skin over the inner corners of the eyes
• Show deficits in cognitive development and
motor development
Genetic Counseling and Prenatal Testing
• Genetic counselors compile information about a
couple’s genetic heritage to explore if their children will
have a genetic abnormality.
• Couples with likelihood of passing on genetic
abnormality tend to adopt or not have children of their
own.
• Prenatal testing can indicate if the embryo or fetus is
carrying genetic abnormalities.
Amniocentesis
• Performed on mother 14-16 weeks after conception
• Syringe withdraws fluid from the amniotic sac; contains
cells sloughed off by fetus
• Cells separated, grown in culture, and examined for
genetic and chromosomal abnormalities
• Routine for women over 35 to detect for Down
syndrome; other chromosomal abnormalities increase
dramatically as women approach 40
• Amniocentesis carries some risk of miscarriage.
Chorionic Villus Sampling
• Carried out between 9th and 12th week of pregnancy
• Syringe inserted through vagina into uterus to suck out
threadlike projections (villi) from the outer membrane
that covers the amniotic sac and fetus
• Results available in days
• CVS slightly greater risk than amniocentesis of
miscarriage; both increase the risk of miscarriage
Ultrasound
• Sound waves that are too high in frequency to be heard
by human ear obtain information about the fetus
• Ultrasound waves are reflected by the fetus; computer
uses the information to generate a picture of the fetus
• Picture is termed a sonogram
• Used to guide the syringe in amniocentesis and CVS by
determining the position of the fetus
• Used to track growth of fetus, detect multiple
pregnancies, detect structural abnormalities
Blood Tests
• Used to identify sickle-cell anemia, Tay-Sachs disease,
and cystic fibrosis
• Alpha-fetoprotein (AFP) used to detect neural tube
defects such as spina bifida and chromosomal
abnormalities
• Neural tube defects cause elevation in the AFP level in
the mother’s blood
• Elevated AFP levels related to increased risk of fetal
death
Heredity and the Environment
Heredity and Environment
• Inheritance, nutrition, learning, exercise, accident, and
illness contribute to development of traits
• Genotypes
-Set of traits we inherit from our parents
• Phenotypes
-Actual set of traits
-Both genetic and environmental influences
Kinship Studies
• The more closely people are related, more genes they
have in common
• Parents and children have 50% genetic overlap
• Siblings have 50% genetic overlap
• Aunts, uncles have 25% overlap with nieces and
nephews; grandparents, 25% overlap with grandchildren
• First cousins have 12.5% overlap
Twin Studies
• Monozygotic (MZ) twins share 100% of genes
-MZ twins resemble each other more closely than
DZ twins on a number of physical and
psychological traits.
-MZ twins more likely to look alike and be similar in
height
-MZ twins more likely to share autism, depression,
schizophrenia, and vulnerability to alcoholism
• Dizygotic (DZ) twins share 50% of genes; same as other
siblings
Adoption Studies
• Children that are separated from their natural parents at
an early age and reared by adoptive parents provide
special opportunities for sorting out nature and nurture.
• When children who are reared by adoptive parents are
nonetheless more similar to their natural parents in a
trait, a powerful argument is made for a genetic role in
the appearance of that trait.
– Examples: Psychological disorders, addiction
Conception: Against All Odds
Conception
• Conception refers to the union of an ovum and a sperm
cell.
• Ova
– Women born with all the ova they will ever have, about 400,000
– Released from ovarian follicle and enter fallopian tube; 3-4 days
later, egg propelled by small, hairlike structures called cilia, and
perhaps, by contractions in the wall of the tube
• If egg not fertilized, discharged through the uterus and
vagina, along with endometrium that had formed to
support an embryo, in the menstrual flow; during
reproductive years, about 400 ova will ripen and be
released
Conception (cont’d)
• Ova are larger than sperm, barely visible to the eye
• Sperm cells develop through stages; sperm with Y sex
chromosomes swim faster than sperm with X sex
chromosomes.
• Male fetuses suffer a higher rate of miscarriage than
females, often during the first month of pregnancy.
• 150 million sperm ejaculated, only 1 in 1,000 can
approach an ovum
Infertility
• Infertility problems with men
- Low sperm count (most common)
- Lack of sperm
- Genetic factors
- Environmental poisons
- Diabetes
- Sexually transmitted infections
- Overheating of the testes
- Pressure (e.g., bike seats)
- Aging
- Certain prescription and illicit drugs
Infertility (cont’d)
• Infertility problems with women
- Irregular ovulation, lack of ovulation
- Irregularities among the hormones that govern
ovulation, stress, and malnutrition
- Pelvic inflammatory disorder (PID)
-infection that scars the fallopian tubes and other organs,
impeding the passage of sperm
-Endometriosis
-can obstruct the fallopian tubes
Infertility Options
• Artificial insemination
-Sperm is collected and quick-frozen
-Sperm then injected into woman’s uterus at time of ovulation;
addresses low sperm count as well as low sperm motility
-Can be used for a woman to get pregnant who does not have
a partner
• In vitro fertilization
-Ripened ova are surgically removed from the mother and
placed in laboratory dish; father’s sperm also placed in dish
-One or more ova fertilized and injected into mother’s uterus
to become implanted
-Used when fallopian tubes are blocked or father has low
sperm motility
-A donor can be used
Infertility Options (cont’d)
• Donor IVF
-Mother does not produce ova
-Ovum from another woman is fertilized and injected into the
uterus of the mother-to-be
• Surrogate mothers
-Mothers who bring baby to term for other women who are
infertile
-Can be artificially inseminated by partner of infertile woman
• Adoption
-Another way for people to obtain children that results in the
formation of loving new families
Preimplantation Genetic Diagnosis (PGD)
• Reliable method for selecting the sex of the child prior to
implantation
• Ova are fertilized in vitro
• After a few days of cell division, cell is extracted from
each
• Sex chromosomal structure of the cell is examined
microscopically to determine its sex
• Embryos of desired sex are implanted in the woman’s
uterus
Prenatal Development
Prenatal Development
• Normal gestation period 280 days; from date of
fertilization 266 days
• Divided into three periods
1. Germinal stage (Weeks 0 - 2)
2. Embryonic stage (Weeks 3 - 8)
3. Fetal stage
(Weeks 9 - Birth)
The Germinal Period
•
•
•
•
First two weeks after conception
Creation of the fertilized egg (the zygote)
Cell division
Implantation -- the attachment of the zygote to
the uterine wall
– takes place about 10 to 14 days after conception
The Embryonic Period
• Occurs from two to eight weeks after
conception
• Rate of cell differentiation intensifies
• Every body part eventually develops
– Neural tube
– Organogenesis -- the process of organ formation
during the first two months of prenatal development
• organs are especially vulnerable to environmental
influences
The Fetal Period
• Fetal period begins two months after conception
(normal gestation is 38-40 weeks)
• Three months after conception -- fetus is about
3 inches long; weighs about 3 ounces
• By the end of the fifth month, the fetus is about
12 inches long and weighs close to a pound
– Vernix
– Lanugo
• At birth, the average American baby weighs 7½
pounds and is about 20 inches long
Three Trimesters
• The germinal and embryonic periods occur in
the first trimester
• The fetal period begins toward the end of the
first trimester and continues through the second
and third trimesters
• Age of Viability (the chances of surviving
outside the womb) occurs at the beginning of
the third trimester
– About 24-25 weeks after conception
Environmental Influences on Prenatal
Development
• Maternal malnutrition effects
-low birth weight
-prematurity
-retardation of brain development
-cognitive deficiencies
-behavioral problems
-cardiovascular disease
• Fetal malnutrition can sometimes be overcome by a supportive,
care-giving environment.
• Supplementing diets of pregnant women shows positive effects on
motor development of infants.
• Maternal obesity linked with higher risk of stillbirth
Environmental Influences on Prenatal
Development (cont’d)
• Women should gain between 25-35 pounds
-Overweight women may gain less
-Slender women may gain more
• Teratogens
-Environmental agents can harm the embryo or fetus
-Includes drugs taken by mother, lead, mercury
• Pathogens
-Disease-causing organisms
-Bacteria and viruses
Environmental Influences on Prenatal
Development (cont’d)
• Critical periods refer to the times when organs are
developing.
• Particular teratogens at a particular time can be harmful
to the fetus.
• Sexually transmitted infections such as syphilis and
HIV/AIDS can affect the development of the fetus.
-Routine blood tests are given early in pregnancy to diagnose
syphilis
Drugs Taken by the Parents (cont’d)
Alcohol
• Heavy alcohol consumption during pregnancy can result in fetal
alcohol syndrome (FAS).
• FAS babies
-often smaller, with smaller brains
-facial features include widely spaced eyes, underdeveloped upper jaw,
flattened nose
• Psychological characteristics appear to reflect dysfunction of the
brain.
• Maladaptive behaviors such as poor judgment, distractibility, and
difficulty perceiving social cues are common.
Drugs Taken by the Parents (cont’d)
Caffeine
• Research regarding caffeine consumption is
inconsistent
-Several studies have found that pregnant women who take in
a good deal of caffeine are more likely than nonusers to have
a miscarriage or a low-birth-weight baby.
Drugs Taken by the Parents (cont’d)
• Cigarettes
– Consist of nicotine, carbon monoxide, and hydrocarbons (tars)
-Nicotine and carbon monoxide pass through placenta and reach the fetus
-Nicotine stimulates the fetus; long-term effects unknown
-Carbon monoxide decreases amount of oxygen available to the fetus
-Connected with impaired motor development academic delays, learning
disabilities, mental retardation, and hyperactivity
• Smokers’ babies likely to be smaller than those of nonsmokers
-Babies of smokers more likely to be stillborn or to
die soon after birth
• Men who smoke are more likely to produce abnormal sperm.
-Babies of fathers who smoke have higher rates of birth
defects, infant mortality, lower birth weights, and
cardiovascular problems.
Parents’ Age
• Older fathers more likely to produce abnormal sperm
• 20’s ideal age for women to bear children
• Teenage pregnancy can result in higher incidence of
infant mortality and low birth weight.
• Stillborn or preterm babies increase as age of mother
increases; adequate prenatal care decreases this
likelihood even for first-time older mothers.