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Prenatal Development

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Prenatal Development
345
Beginnings
The development of human behavior is shaped by
both heredity and environment—by nature and nurture. The joint and inseparable influence of these two factors in
development is perfectly illustrated in the
case of professional golfer Tiger Woods,
shown here as a youngster with his
father, who not only provided some of
Tiger’s genes but also served as his golf
teacher.
A TIGER IN TRAINING
influenced by genes than by the environment. Does this mean that a person who is six
feet tall grew, say, four of those feet because of genes and the other two feet because of
environment? No. It means that more of the variability in height that we see among
people can be explained by the genetic differences among them than by the environmental differences. In fact, genes do account for about 80 to 95 percent of the variability in height. So if a person is taller or shorter than average, genetic factors are probably the primary cause. We say “probably” because determination of a genetic influence
on height refers only to the origins of average individual differences in the population.
So even though the differences in people’s heights are due mainly to genetic factors,
a particular person’s height could be due mainly to an early illness or other growthstunting environmental factors.
To see how the logic of behavioral genetics applies to conclusions about psychological characteristics, suppose a researcher discovered that a certain personality trait is
50 percent heritable. This finding would mean that approximately half of the differences
between people on that trait can be explained by genetic factors. It would not mean that
each person inherits half of the trait and gets the other half from environmental influences. In other words, the results of behavioral genetics research allow us to draw general conclusions about the influence of nature and nurture on certain characteristics, but
those conclusions do not necessarily apply to the origins of a particular person’s characteristics. Keep in mind, too, that the effects of genes on our traits and behaviors are not
always simple or fixed. Complex traits such as intelligence and personality are influenced
by many genes, as well as by many environmental factors. So genetic influence means
just that: influence (Plomin et al., 2002). Genes can affect a trait without completely
determining whether that trait will actually appear in a particular individual.
Genes and the Environment
The relative contributions of nature and nurture differ for specific aspects of development, but their influences on all human characteristics are forever intertwined. They
are also mutually influential. Just as the environment encourages or discourages the
expression of an individual’s inherited characteristics, those inherited characteristics
determine the individual’s environment to some extent. In short, heredity creates predispositions that interact with the immediate environment, including family and teachers, books, and computers (Caspi et al., 2002). This interaction is what produces developmental outcomes. Let’s now consider how it all begins.
Beginnings
䉴 Why should pregnant women stay away from tobacco and alcohol?
Nowhere are the intertwined contributions of heredity and environment clearer than
during the eventful nine months before birth, when a single fertilized egg develops into
a functioning newborn infant.
Prenatal Development
chromosomes Long, thin structures in
every biological cell that contain genetic information in the form of genes.
genes Hereditary units, located on
chromosomes, that contain biological
instructions inherited from both parents, providing the blueprint for physical development.
deoxyribonucleic acid (DNA) The molecular structure of a gene that provides
the genetic code.
The process of development begins when sperm from a father-to-be fertilizes the egg
of a mother-to-be and forms a brand-new cell. Most human cells contain forty-six
chromosomes (pronounced “KROH-muh-sohmz”), arranged in twenty-three matching pairs. Each chromosome is made up of thousands of genes, the biochemical units
of heredity that govern the development of an individual. Genes, in turn, are composed
of deoxyribonucleic acid (DNA). (Deoxyribonucleic is pronounced “dee-OKS-ee-ryeboh-noo-KLAY-ic.”) The DNA in genes provides coded messages that serve as blueprints for constructing every aspect of a physical human being, including eye color,
height, blood type, inherited disorders, and the like. All of this information fits in less
space than the period that ends this sentence.
346
A FETUS AT TWELVE WEEKS In this
photo of a fetus at three months after
conception, the umbilical cord and placenta are clearly visible. At this point in
prenatal development, the fetus can kick
its legs, curl its toes, make a fist, turn its
head, squint, open its mouth, swallow,
and take a few “breaths” of amniotic
fluid.
Chapter 9
Human Development
New cells in the body are constantly being produced by the division of existing cells.
Most of the body’s cells divide through a process called mitosis (pronounced “myeTOH-sis”), in which the cell’s chromosomes duplicate themselves so that each new cell
contains copies of the twenty-three pairs of chromosomes in the original cell.
A different kind of cell division occurs when a male’s sperm cells and a female’s egg
cells, called ova, are formed. This process is called meiosis (pronounced “mye-OH-sis”).
In meiosis, the chromosome pairs are not copied. Instead, they are randomly split and
rearranged, leaving each new sperm and egg cell with just one member of each chromosome pair, or twenty-three single chromosomes. No two of these special new cells
are quite the same, and none contains an exact copy of the person who produced it.
So, at conception, when a male’s sperm penetrates, or fertilizes, the female’s ovum, a
truly new cell is formed. The fertilized cell, called a zygote, carries the usual twentythree pairs of chromosomes, but half of each pair comes from the mother and half
from the father. The zygote represents a unique heritage—a complete genetic code for
a new person that combines randomly selected aspects from both parents. The zygote
divides first into copies of itself; then it divides and redivides into the billions of specialized cells that form a complete new human being.
Stages of Prenatal Development The first two weeks after conception are called
the germinal stage of development. By the end of this stage, the cells of the dividing
zygote have formed an embryo (pronounced “EM-bree-oh”). Next comes the embryonic stage of development, during which the embryo quickly forms a heart, nervous
system, stomach, esophagus, and ovaries or testes. By two months after conception,
when the embryonic stage ends, the embryo looks decidedly human, with eyes, ears,
nose, jaw, mouth, and lips. The tiny arms have elbows, hands, and stubby fingers; the
legs have knees, ankles, and toes.
The seven-month period remaining until birth is called the fetal stage of prenatal
development. During this stage, the various organs grow and start to function. By the
end of the third month, the fetus can kick, make a fist, turn its head, open its mouth,
swallow, and frown. In the sixth month, the eyelids, which have been sealed, open. The
fetus is now capable of making sucking movements and has a well-developed grasp,
taste buds, eyebrows, and eyelashes.
By the end of the seventh month, the organ systems, though immature, are all functional. In the eighth and ninth months, the fetus can respond to light and touch, and
it can hear what is going on outside. It can also learn. When it hears its mother’s familiar voice, its heart beats a little faster, but the heart slows if the fetus hears a stranger
(Kisilevsky et al., 2003).
Prenatal Risks During prenatal development, a spongy organ called the placenta
embryo The developing individual
from two weeks to two months after
fertilization.
fetus The developing individual from
the third month after conception until
birth.
teratogens Harmful substances, such
as alcohol and other drugs, that can
cause birth defects.
critical period An interval during
which certain kinds of growth must
occur if development is to proceed
normally.
appears and attaches itself to the mother’s uterus through an umbilical cord. (The cord
is detached at birth, but you can see where yours was by looking at your navel.) The
placenta sends nutrients from the mother to the developing baby and carries away
wastes. It also screens out many potentially harmful substances, including most bacteria. This screening is imperfect, however: Gases, viruses, nicotine, alcohol, and other
drugs can pass through. Severe damage can occur if the baby’s mother takes certain
drugs, is exposed to certain toxic substances such as mercury, or has certain illnesses
while organs are forming in the embryonic stage (Koger, Schettler, & Weiss, 2005).
Harmful external substances that invade the womb and result in birth defects are
called teratogens (pronounced “tuh-RAT-uh-jens”). Teratogens are especially damaging in the embryonic stage, because it is a critical period in prenatal development, a
time during which certain kinds of growth must occur if development is to proceed
normally. If the heart, eyes, ears, hands, and feet do not appear during the embryonic
stage, they cannot form later on. If they form incorrectly, the defects will be permanent. So even before a mother knows she is pregnant, she may accidentally damage her
infant by exposing it to teratogens. For example, a baby whose mother has rubella
(German measles) during the third or fourth week after conception has a 50 percent
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