Four Themes of This Chapter
1.
2.
3.
4.
Genes are one part of a complex
developmental system.
Development involves differentiation.
Development involves repeated
reorganization and qualitative change.
New structures and capacities emerge
in an orderly way from those that
existed before.
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Prenatal period:
The period of development prior to birth.
Differentiation:
A developmental process
in which structures and
functions become
increasingly specialized.
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Genetic Processes
Chromosomes: Threadlike structures
in the nucleus of cells, composed
of long molecules of DNA.
Genes: Segments of DNA that
contain the code for producing a
particular protein.
A single chromosome may have as many as
20,000 genes.
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Mechanisms of Cell Division
Somatic cells: The cells that make up the
body, not including egg and sperm.
Mitosis: The process of cell division by
which the body grows and repairs itself,
in which genetic material from the
parent cell is duplicated in each
daughter cell.
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The process of mitosis:
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DNA in the original cell duplicates itself,
producing double-stranded chromosomes.
Double-stranded chromosomes line up along
the cell’s center; each one splits into two
single-stranded chromosomes.
Single-stranded chromosomes separate and
move to opposite sides of the cell.
The original cell divides down the middle,
producing two new cells that are exact copies
of the original, with 46 chromosomes.
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Homologues: Two chromosomes that form
one of the 23 pairs of human
chromosomes and resemble each other
in size, shape, and the types of genes
they carry.
Meiosis: The
process of cell
division by
which egg and
sperm cells are
formed.
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Meiosis
Germ cells: The cells from which eggs
and sperm are produced.
• The DNA in the germ cells duplicates itself.
• Homologous chromosomes arrange themselves in pairs, one
from each parent.
• These chromosomes then separate and move to opposite ends
of the cell.
• The cell divides; each of the two resulting cells has 23 doublestranded chromosomes (first meiotic division).
• The double-stranded chromosomes in these two cells split.
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Meiosis
These cells divide, resulting in four cells, each
with a single set of 23 chromosomes (second
meiotic division).
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For males, all four cells become mature sperm.
For females, only one becomes a mature egg.
Gamete: A mature reproductive cell.
Egg or sperm.
Random assortment: Shuffling of
chromosomes from the mother and
the father during meiosis.
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Crossing over:
An exchange of corresponding segments of
genetic material between homologous
chromosomes during meiosis.
The chance that a couple will produce two
identical children from separate sperm-egg
combinations is 1 in 64 trillion.
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How Genes Influence Development
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Development is the result of an interplay
between genes and environment.
Genes are turned on or off by chemical
signals in the cell.
Environmental factors (nutrients, drugs,
viruses, etc.) affect the cell.
Genes are influenced by the organism’s
developmental history.
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The Interaction of Genes and Environment:
The Case of Physical Gender Development
Sex chromosomes: In humans, the 23rd pair
of chromosomes, which determine genetic
gender.
Females normally have two X chromosomes.
Males, normally one X and one Y chromosome.
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Critical
period
A limited time when some part of a
developing organism is susceptible
to influences that can bring about
specific and permanent changes.
Gonads
The sex glands: The ovaries and
testes.
Hormone
A chemical produced in the body
that regulates physiological
processes.
Androgens Male sex hormones.
Estrogens
Female sex hormones.
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How Genes Affect One Another
Allele
One of several alternate forms of
a particular gene.
Homozygous
Carrying two identical alleles for a
particular trait.
Heterozygous Carrying two different alleles for a
particular trait.
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How Genes Affect One Another
Genotype
An individual’s genetic makeup.
Phenotype
An individual’s observable traits.
Sex-linked
traits
Recessive genetic traits that are
carried on the X chromosome and
are commonly expressed only in
males.
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Conception
The joining of sperm and egg depends on
successful completion of a complicated chain of
events.
 Ovulation: Ovum (egg cell) is released into
one of the fallopian tubes (the passages that
lead into the uterus.
 Journey down fallopian tube takes days.
 If sexual intercourse occurs at the appropriate
time, the ovum encounters thousands or
million of sperm.
 If a sperm penetrates the ovum’s outer
member, a zygote is formed.
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Conception
Zygote: The cell resulting from the
union of a sperm cell with an ovum.
Twins can develop if:
• Two eggs are released by the ovaries at the same time,
• or one fertilized egg splits into two embryos.
Dizygotic twins: Fraternal twins, the result of
fertilization of 2 ova by different sperm.
Monozygotic twins: Identical twins, from the
division of one zygote into 2 separate units.
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The Stages of Prenatal Development
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Germinal period (conception through week 2):
A tiny, self-contained cluster of cells becomes
implanted in the lining of the mother’s uterus.
Cell differentiation begins.
Embryonic period (weeks 3-8): Major organs
and body parts develop.
Fetal period (week 9 to birth): Organism
greatly increases in size and becomes moving,
sleeping, waking being.
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The Germinal Period:
Conception Through Week 2
Blastocyst
Hollow, ball-like structure into
which zygote develops in 1st week.
Embryoblast Group of cells at one end of the
blastocyst; develops into embryo.
Trophoblast
Cells in blastocyst that form basis
of embryo’s life-support system.
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The Embryonic Period:
Weeks 3 Through 8
Embryo
The developing organism during weeks 3 through 8
of prenatal development.
Organogenesis The formation of organs and other major body
structures.
Placenta
The Embryo’s Life-Support System
A mass of tissue that supplies oxygen and nutrients
to embryo, carries away waste products.
Umbilical cord
A cord containing blood vessels that connects the
embryo with the placenta.
Amniotic sac
Fluid-filled sac that surrounds and protects the
embryo and the fetus.
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Embryonic Cell Differentiation
Endoderm
Mesoderm
Cells that develop into internal
organs such as the stomach, liver,
and lungs.
Cells that become the muscles,
skeleton, and blood.
Ectoderm
Cells that form the central nervous
system, sensory organs, and skin.
Embryonic
induction
Chemical interaction between cells
of different tissues; triggers
developmental changes.
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The Timetable for Embryonic Development
Cephalocaudal
development:
The principle that
development
proceeds from the
head downward.
Proximodistal
development:
The principle that
development
proceeds from the
center of the body
outward.
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The Fetal Period:
Week 9 to Birth
Fetus:
The developing organism during weeks
9 through 38 of prenatal development.
During this period:
• The fetus grows rapidly.
• Structures become refined.
• The fetus becomes increasingly active
and responsive to environment.
• Behaviors become increasingly regular
and integrated.
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A Closer Look:
Prenatal Brain Development
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Basic structures of the central nervous
system form during embryonic period:
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18 days after conception, CNS begins
formation as ectoderm thickens and begins
to fold in upon itself.
At 21-22 days, folds in ectoderm grow
together to form neural tube.
At week 4, neurons are differentiating, and
brain and spinal cord have started to form.
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A Closer Look:
Prenatal Brain Development
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During fetal period, neurons form rapidly
and brain structures continue to develop:
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In weeks 10-20, most neurons of the
cerebral cortex form.
By week 24, synapses begin to form.
At birth, volume of infant’s cerebral cortex
is 1/3 that of an adult’s, even though it
contains almost all the neurons it will ever
have.
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The Mother’s Experience of Pregnancy
Trimesters:
Three-month periods that correspond to changes
in the mother’s experience of pregnancy.
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Fatigue, drowsiness, swelling, frequent urination, morning
sickness are common during the first trimester.
Intensity of symptoms varies from woman to woman.
Second trimester often the most enjoyable for the
mother.
Quickening: The first fetal movements, usually
felt by mother by end of the fourth month.
Third trimester can be trying, as the increase in fetal size
pressures her organs.
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Problems in Prenatal Development
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Congenital (birth) defects: Any abnormality
present at birth.
Genetic defects
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Single-gene (Mendelian) disorder: Any disorder
produced by a single gene.
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Chromosomal abnormality: Errors in meiosis produce
sperm or egg cells with incorrect numbers of
chromosomes or damaged chromosomes.
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example: Huntington disease.
example: Down Syndrome (trisomy 21).
Sex chromosome abnormalities: When baby receives
an abnormal number of sex chromosomes.
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examples: Klinefelter and Turner syndromes.
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Sex Chromosome Abnormalities
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XXY
Klinefelter syndrome: Males may receive an extra X
chromosome.
XYY
XYY syndrome: Males may receive an extra Y
chromosome.
XXX
Trisomy X: Females may receive an extra X
chromosome.
X0
Turner syndrome: Females may have only one sex
chromosome.
Many never know they have sex chromosome problems.
However, these abnormalities may result in:
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infertility
educational problems
reduced IQ
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Environmental Influences
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alcohol, tobacco, illicit drugs
medications
diseases
maternal stress
maternal malnutrition
maternal age
Teratogen:
Any substance in the environment that can cause physical
malformations during prenatal development.
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Fetal Alcohol Syndrome
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1 in 3 babies born to heavy drinkers
suffer FAS.
Unusual facial characteristics, poor
growth, and central nervous system
problems.
Lower levels of alcohol can cause
neurological and behavioral problems.
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Detection & Treatment of Fetal Disorders
Ultrasound
A technique that produces a computer image of a
fetus by bouncing sound waves off it.
Amniocentesis
Withdrawal of amniotic fluid through a needle
inserted into the mother’s abdomen to check for
chromosomal abnormalities.
Chorionic villus A technique for analyzing fetus’s genetic makeup;
sampling
cells are suctioned from developing placenta
through small tube through vagina and cervix or
through abdominal wall.
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The Stages of Labor
1.
2.
3.
Regular, strong contractions begin 15-20 min. apart;
become stronger, more frequent as labor progresses,
till cervix is fully opened (dilated). From an hour to
more than a day.
Normal, head-first deliveries begin 2nd stage with
crowning, when crown of infant’s head pushes
through cervix into vagina. 30 min. to 2 hrs.
Baby is delivered. Ends with delivery of afterbirth,
which includes placenta and other membranes.
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Apgar Scale
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Apgar scale – score (from zero to 10)
at one and five minutes after birth is
the sum of the ratings (0, 1, 2) for
heart rate, respiration, reflexes, muscle
tone, and skin color.
Scores below 5 indicate the need for
immediate medical attention.
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Anoxia:
A disruption in the baby’s oxygen supply
during or just after birth.
Can occur in two basic ways:
1.
Umbilical cord may become pinched
during delivery.
2.
In breech-birth, infant is delivered in
bottom- or feet-first position.
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Low Birth Weight and Prematurity
Premature:
Born less than 35 weeks after conception.
Low birth weight:
Weighing less than 2500 grams at birth.
Very low birth weight:
Weighing less than 1500 grams at birth.
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Cultural Variation in Childbirth
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In Holland, most births take place at home
with a midwife.
Lay midwives deliver many babies in
developing countries.
The U.S. has changed somewhat over the past
30 years:
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Childbirth preparation classes, e.g. Lamaze method
Decreased anxiety about labor and delivery
Less reliance on painkillers
Increase in deliveries by cesarean section.
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Prenatal Development in Context
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Genes guide within environmental
contexts that include events and
developmental changes in the
past and present.
Genes are activated and
deactivated in response to
environmental signals.
The mother is embedded in
physical, social, and cultural
environment that can affect her
body and her developing child.
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