Genetics & Prenatal
Development
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
Sex Linked Traits
• The sex chromosomes, the 23rd pair of chromosomes,
determine biological sex
• In females, the 23rd pair of chromosomes is made up of
two large X chromosomes. XX
• In males, a large X chromosome and a smaller Y
chromosome make up the 23rd pair. XY
• For males, the smaller Y chromosome often does not
contain a corresponding gene segment to match the one
on the X chromosome.
• This means that a male can display certain recessive
characteristics as the result of having only one recessive
gene carried on the X chromosome of his XY pair.
• Traits determined by recessive genes on the X
chromosomes are referred to as sex-linked recessive
characteristics
Physical and 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
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,
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
• 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
• 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
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.
Prenatal Development – 45 Days
Prenatal Development – 2
months
See the
changes that
occur in
prenatal
development
in this quick
video
Prenatal Influences
on 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
and PCBs
– Drugs taken by the mother, such as alcohol,
nicotine, cocaine, and heroin
Prenatal Development
• Play “Teratogens and Their Effects on
the Developing Brain and Mind”
(12:44) Segment #12 from The Mind:
Psychology Teaching Modules (2nd
edition)
Smoking and Birth Weight
Fetal Alcohol Syndrome
– cluster of defects occurring in infants born to
mothers that drink heavily during pregnancy
– leading cause of mental retardation
– can be totally prevented by abstaining from
alcohol during pregnancy
Prenatal Brain Development
• Play “The Effects of Hormones and
the Environment on Brain
Development” (6:50) Module #2 from
The Brain: Teaching Modules (2nd
edition)