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.
Mitosis
• Life begins as a single cell or zygote that divides
repeatedly.
• Mitosis
– Genetic code carried into new cells in our bodies
• DNA breaks apart; double helix duplicates
• DNA forms two camps on either side of the cell, and
then the cell divides
Fig. 2-1, p. 24
Fig. 2-2, p. 25
Meiosis
• Sperm and ova are produced through meiosis or
reduction division.
• 46 chromosomes within the cell nucleus first line up into
23 pairs.
• DNA ladders unzip, leaving unpaired halves of
chromosome; when cell divides, one member of each
pair goes to each newly formed cell.
• Each new cell nucleus contains only 23 chromosomes,
not 46.
Fig. 2-3, p. 25
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
Dominant and Recessive Traits
• Traits determined by pairs of
genes
– each member of pairs of
genes termed an allele
• Dominant trait
– trait that is expressed
• Recessive trait
• Homozygous
– having two identical alleles
– trait that is not expressed
when genes have been paired
with dominant genes
• Heterozygous
– having two different alleles
• Incomplete
dominance/codominances
– when both alleles are shown
• Carriers
– people who bear one
dominant gene and one
recessive trait
Fig. 2-4, p. 26
Table 2-1, p. 27
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
Fig. 2-5, p. 28
Sex-Linked Chromosomal Abnormalities
MALES
FEMALES
• 1 male in 700-1,000 has XYY • 1 female in 2,500 has a single X
– leads to heightened male
resulting in Turner Syndrome
secondary sex characteristics
– most individuals with disorder are
infertile
– poorly developed ovaries, short,
and infertile.
• 1 male in 500 has Klinefelter • 1 female in 1,000 has XXX sex
chromosomal structure called
syndrome (XXY)
– leads to inadequate development Triple X syndrome
of male primary and secondary
sex characteristics
– normal in appearance but tend to
show lower-than-average language
skills and poorer memory for recent
events
Klinefelter Syndrome (XXY)
• 1 male in 500
• Caused by extra X on sex chromosome
• Produces less testosterone than normal males
• Characteristic features of testes, deepening of voice,
musculature, and male pattern of body hair do not
develop properly
• Typically treated with testosterone replacement therapy
Turner Syndrome (X)
• 1 female in 2,500
• Single X chromosome
• External genitals are normal, ovaries poorly developed,
producing little estrogen
• Shorter than average and infertile
• Cognitive deficits with low estrogen: problems with
visual-spatial skills, mathematics, and nonverbal
memory
Triple X Syndrome (XXX)
• 1 female in 1,000
• Appears normal, but demonstrates lower-than-average
language skills
• Poorer memory for recent events
• Development of external sex organs appears normal
• Increased incidence of infertility
Phenylketonuria (PKU)
• Enzyme disorder transmitted by a recessive gene
affecting 1 child in 8,000
• Cannot metabolize an amino acid called phenylalanine;
builds up in body and impairs functioning of the central
nervous system (CNS)
• Results are mental retardation, psychological disorders,
physical problems
• No cure, but children can be placed on diets low in
phenylalanine and develop normally
Huntington’s Disease (HD)
•
•
•
•
•
•
•
•
Affects 1 in 18,000 Americans
Fatal, progressive degenerative disorder
Dominant trait
Physical symptoms include uncontrollable muscle
movements
Psychological symptoms include loss of intellectual
functioning and personality change
Onset of HD is delayed until middle adulthood
Possibly half their offspring will develop it
Medicines can deal with some symptoms
Sickle-Cell Anemia
• Carriers
-1 in 10 African Americans
-1 in 20 Latino or Latina Americans
• Caused by a recessive gene
• Red blood cells take on the shape of a sickle and clump
together, obstructing small blood vessels and
decreasing the oxygen supply
• Lessened oxygen can impair cognitive skills
• Physical problems
-painful and swollen joints
-jaundice
-potentially fatal conditions such as pneumonia, stroke, and
heart and kidney failure
Tay-Sachs Disease
• 1 in 30 Jewish Americans are carriers
• Caused by recessive gene
• Causes the central nervous system to degenerate,
resulting in death
• Commonly found among children in Jewish families of
Eastern European background
Cystic Fibrosis
• Approx. 30,000 Americans have disorder, 10 million
more are carriers (1 in 31 people)
• Caused by recessive gene
• Most common fatal hereditary disease among
Caucasians
• Children suffer from excessive production of thick
mucus that clogs the pancreas and lungs
• Most victims die of respiratory infections in their 20s
Sex-Linked Genetic Abnormalities
• Genetic defects only carried on the X sex chromosome
• Hemophilia – genetic disorder in which blood does not
clot properly
• Duchenne – muscular dystrophy; sex linked
• Involve recessive genes
• Females with two X sex chromosomes are less likely
than males to show sex-linked disorder
• Sons of female carriers are more likely to be afflicted
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.
Fig. 2-6, p. 30
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.
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
Conception (cont’d)
• Sperm have to fight vaginal acidity, gravity, and swim
against current fluid coming from the cervix.
• If sperm survive, reach fallopian tubes 60-90 minutes
after ejaculation; approximately 2,000 enter the correct
tube
• Sperm are “egged on” by a change in calcium ions that
occurs when an ovum is released.
• Only one enters; have to thin the gelatinous layer of
egg; secrete an enzyme that briefly thins layer
• Once sperm enters, layer thickens and locks out other
sperm
Conception (cont’d)
• Chromosomes from the sperm cell line up across from
corresponding chromosomes in the egg cell.
• Form 23 new pairs with unique set of genetic
instructions
• The direction that sperm travel is guided by a change in
calcium ions that occurs when an ovum is released.
Fig. 2-8, p. 33
Fig. 2-9, p. 34
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 Stage
• The period from conception to implantation
• Blastocyst
-Cells separate into groups that will become different
structures
-Inner part of blastocyst has two distinct layers that form a
thickened mass of cells called embryonic disk
-These cells eventually become the fetus
• Trophoblast
– Four membranes that will protect and nourish the embryo
1. One produces blood cells, then it disappears
2. One develops into umbilical cord and blood vessels of the
placenta
3. One develops into the amniotic sac
4. One becomes the chorion, which will line the placenta
The Embryonic Stage
• Development follows cephalocaudal (head to tail) and proximodistal
(near to far).
• Growth of the head takes precedence over growth of the lower
parts of the body.
• Relatively early maturation of the brain and organs that lie near the
spine
• Ectoderm
-Outer layer of cells
-Develops into nervous system, sensory organs, nails, hair,
teeth, and outer layer of skin
• Endoderm
-Forms digestive and respiratory system, liver, and pancreas
• Mesoderm
-Develops into excretory, reproductive, and circulatory
systems, muscles, the skeleton, and the inner layer of skin
The Embryonic Stage (cont’d)
• Head and blood vessels form third week after
conception
• Heart starts beating
• Major organ systems develop within first 2 months
-Arm buds and legs appear by end of 1st month
-Eyes, ears, nose, and mouth take shape
-Nervous system and brain begins to develop
• During 2nd month, nervous system begins to send
messages
• Embryo is 1 inch long and weighs 1/30th of an ounce
• Teeth buds are formed by end of stage
The Embryonic Stage (cont’d)
• At 5-6 weeks, internal and external genitals resemble
primitive female structures.
• By 7th week, genetic code asserts itself, causing sex
organs to differentiate
• Genetic activity on Y sex chromosome causes the
testes to begin to differentiate.
• Ovaries begin to differentiate if the Y chromosome is
absent.
• Androgens begin to produce.
• Embryo and fetus develop within protective amniotic
sac; sac is surrounded by clear membrane and contains
amniotic fluid
The Embryonic Stage (cont’d)
• Amniotic fluid acts as natural air bag allowing embryo
and fetus to move without injury.
• Placenta is a mass of tissue that permits the embryo to
exchange nutrients and waste with the mother.
• Mother and embryo have separate circulatory systems.
• Placenta permits oxygen and nutrients to reach the
embryo from the mother, and permits carbon dioxide
and waste products to pass to the mother from the
embryo.
The Embryonic Stage (cont’d)
• Mother eliminates waste through her lungs and kidneys.
• Drugs such as aspirin, narcotics, alcohol, tranquilizers,
and others cross the placenta and affect the fetus.
• Placenta secretes hormones that preserve pregnancy,
prepare breasts for nursing, and stimulate contractions.
• Placenta passes from the birth canal after the baby,
hence the term afterbirth.
The Fetal Stage
• Lasts from beginning of third month until birth
• Between 9th and 10th, week fetus responds to external
stimulation
• Major organ systems formed at end of first trimester
• Second trimester consists of further maturation and gain
in size
• Weight advances from 1 ounce to 2 pounds and grows
from about 3 to 14 inches
The Fetal Stage (cont’d)
• Fetus can open and shut eyes, suck thumb at end of
second trimester
• Gains about 5 1/2 pounds and doubles in length
• By 7th month, fetus turns upside down toward delivery
• Doubles in weight by end of 7th month
• 90% survival if born at end of 7th month and given
quality care
The Fetal Stage (cont’d)
• By 13th week, fetus responds to sound waves
• Experiment by DeCasper and Fifer (1980) demonstrated
that a fetus is learning while in uterus
• Fetal movements are noticeable during middle of 4th
month
• At 29-30 weeks, fetus moves limbs vigorously and turns
summersaults
• Fetus gets cramped as it grows, becomes less active
during 9th month
Fig. 2-12, p. 41
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
Environmental Influences on Prenatal
Development (cont’d)
• Rubella
-Women affected by rubella (German measles) during first 20 weeks of
pregnancy stand 20% chance of bearing children with birth defects such
as deafness, mental retardation, heart disease, eye problems (including
blindness)
• Preeclampsia (or Toxemia)
-Life-threatening; characterized by high blood pressure that may afflict
women late in 2nd trimester or early in 3rd trimester
• Rh incompatibility
-Consists of antibodies produced by the mother and transmitted to a fetus
or newborn infant that cause brain damage or death
Drugs Taken by the Parents
• Thalidomide
-A sedative used for treatment of insomnia and nausea
-Causes missing or stunted limbs during second month
of pregnancy
Drugs Taken by the Parents (cont’d)
• Hormones
-Women at risk for miscarriages have been prescribed hormones to
help maintain their pregnancies.
-Progestin
-Synthetic version of naturally occurring progesterone
-Can masculinize the external sex organs of female embryos
-Diethylstilbestrol (DES)
-Used to prevent miscarriage during 1940s and 1950s
-Caused cervical and testicular cancer in some offspring
-1 in 1,000 daughters of DES users will develop cancer in the reproductive
tract
Drugs Taken by the Parents (cont’d)
• High doses of vitamins A and D
-Associated with central nervous system damage, small head
size, and heart defects
• Narcotics (heroine, methadone) easily pass placental
membrane and cause fetuses to become addicted.
-After birth, drug is substituted so serious withdrawal
symptoms are minimized
-Addicted newborns may have behavioral effects,
motoric delays, language delays
Drugs Taken by the Parents (cont’d)
Marijuana
• Contributes to slower fetal growth, low birth weight
• Babies of women who regularly used marijuana show increased
tremors and startling, suggesting immature development of the
nervous system.
• Study by Goldschmidt et al. (2000) indicates prenatal exposure to
marijuana can result in
-increased hyperactivity
-impulsivity
-problems paying attention
-increased delinquency and aggressive behavior
Drugs Taken by the Parents (cont’d)
Cocaine
• Cocaine use during pregnancy increases risk of stillbirth, low birth
weight, and birth defects.
• Infants are excitable, irritable, or lethargic; sleep is disturbed
• Suggestions of delays in cognitive development even at 12 months
of age
• Prenatal exposure linked to lower receptive and expressive
language abilities at older ages
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.
Fig. 2-13, p. 44
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.
Environmental Hazards
• Environmental hazards consist of heavy metals such as
lead, mercury, and zinc.
• Exposure to lead related to delayed mental
development at 1 and 2 years of age
• Consumption of PCB-contaminated fish from Lake
Michigan resulted in babies that were smaller and
showed poorer motor functioning and memory defects.
• Exposure to radiation has been linked to mental
retardation and physical deformity; suggestion to avoid
unnecessary x-rays
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.