Prenatal Development
Module 45
Overview of Genetics
• Chromosomes are long twisted strands
of deoxyribonucleic acid (DNA) and are found in the nucleus of the
cell
• Humans have 23 pairs of chromosomes
• DNA is the chemical basis of heredity and carries instructions
• DNA code carried on each chromosome is arranged in thousands of
segments called genes.
• Genes are the basic unit of heredity
Conception
Conception
• At conception, the genes carried on the 23
chromosomes contributed by your biological
mother’s ovum were paired with the genes
carried on the 23 chromosomes contributed
by your biological father’s sperm, creating
your unique genetic makeup.
• Multiple gene pairs are involved in directing
many complex features of development.
Dominant and Recessive
• Genotype—underlying genetic makeup
• Phenotype—traits that are expressed/displayed
• Dominant genes—will always be expressed if present
– Traits such as freckles, dark eyes, dark hair, and dimples are
referred to as dominant characteristics because they require
only one member of a gene pair to be dominant for the trait to
be displayed
• Recessive genes—will expressed only if paired with an
identical recessive gene. Will not be expressed if paired
with a dominant gene.
• We inherit from our biological parents a genetic
potential, the expression of which can be influenced by
environmental conditions.
Characteristic Dominant
Recessive
Eye color
Brown
Grey, blue
Hair
Dark
Curly
Light
Straight
Hands
Extra fingers 5 fingers
Limb dwarfing Normal limbs
Face
Broad lips
Dimples
Thin lips
No dimples
Just because a trait is Recessive doesn’t mean it’s not common.
Prenatal and
Childhood
Development
Prenatal Development
• Prenatal defined as “before birth”
• Prenatal stage begins at conception and ends with
the birth of the child.
• At conception, chromosomes from the biological
mother and father combine to form a single cell—
the fertilized egg, or zygote.
• The prenatal stage has three distinct phases:
1. germinal period,
2. embryonic period,
3. fetal period.
Prenatal Development
• Conception—when a sperm penetrates the
ovum
• Zygote—a fertilized egg
• Germinal period—first two weeks after
conception
• Embryonic period—weeks three through
eight after conception
• Fetal period—two months after conception
until birth
From Conception to Zygote
Zygote
• A newly fertilized egg
• The first two weeks are a period of
rapid cell division.
• Attaches to the mother’s uterine wall
• At the end of 14 days becomes a
cluster of cells called an embryo.
Embryo
• Developing human from about 14 days until the
end of the eighth week
• Time of rapid growth and intensive cell
differentiation
• Zygote’s inner cells become the Embryo
• Most of the major organs are formed during this
time.
• Genes on the sex chromosomes and hormonal
influences trigger the initial development of the
sex organs
• At the end of the eighth week the fetal period
begins.
Embryo
• The embryo is protectively housed in the fluid-filled
amniotic sac; the embryo’s lifeline is the umbilical
cord.
• Via the umbilical cord, the embryo receives
nutrients, oxygen, and water and gets rid of carbon
monoxide and other wastes
Placenta
• Zygote’s outer cells become the Placenta
• A cushion of cells in the mother by which the
fetus receives oxygen and nutrition
• Acts as a filter to screen out substances that
could harm the fetus
• The umbilical cord attaches the embryo to
the placenta, a disk-shaped tissue on the
mother’s uterine wall.
• The placenta prevents the mother’s blood
from mingling with that of the developing
embryo, acting as a filter to prevent some,
but not all, harmful substances that might be
present in the mother’s blood from reaching
the embryo
Prenatal Development
Prenatal Development – 45 Days
Prenatal Development – 2
months
•
Fetal Period
The period between the beginning of the ninth
week until birth
1. By the end of the third month, the fetus can
move its arms, legs, mouth, and head
2. During the fourth month, the mother experiences
quickening—she can feel the fetus moving.
3. By the fifth month, the
fetus has distinct sleep–
wake cycles and
periods of activity.
4. During the sixth month,
the fetus’s brain activity
becomes similar to that
of a newborn baby.
5. During the final two
months, the fetus will
double in weight.
See the
changes that
occur in
prenatal
development
in this quick
video
Prenatal Factors that Influence
Development
•
•
•
•
•
Nutrition
Anxiety
Mother’s general health
Maternal age
Teratogens—any agent that
causes
a birth defect (e.g., drugs,
radiation, viruses)
Teratogens
• Substances that pass through the placenta’s screen
and prevent the fetus from developing normally
• Teratogens include:
– Exposure to radiation
– Diseases, such as rubella, syphilis, genital herpes, and
AIDS
– Toxic industrial chemicals, such as mercury, lead and
PCBs
– Drugs taken by the mother, such as alcohol, nicotine,
cocaine, and heroin
Smoking and Birth Weight
Fetal Alcohol Syndrome
– cluster of defects occurring in infants born to mothers
that drink heavily during pregnancy
– leading cause of intellectual disabilities
– Epigenetic Effect – alcohol alters the chemical markers
on DNA that switch certain genes on or off.
– can be totally prevented by abstaining from alcohol
during pregnancy
Physical & Psychological
Development Related:
• Physical development begins at conception
• Physical maturity sets limits on psychological
ability
– visual system not fully functional at birth
– language system not functional until much later
• Prenatal environment can have lifetime
influence on health and intellectual ability