Understanding Psychology (10th Ed)

(c)
 
DNA sequence
(d)
Genes
(b)
23 pairs of chromosomes
(a)
Conception
X
X
X
X
X

Y
Female Male 
Baby girl 
Baby boy 
X
X
The 23rd pair 
of chromosomes
of the ovum is 
comprised of 
chromosomes 
XX 
The 23rd pair 
of chromosomes
of the sperm is 
comprised of 
chromosomes 
XY 
X
Y
FIGURE 2
When an egg and a sperm 
meet at the moment of fertilization, the 
egg provides an X chromosome, and 
the sperm provides either an X or a Y 
chromosome. If the sperm contributes 
an X chromosome, the child will have 
an XX pairing on the 23rd chromosome 
and will be a girl. If the sperm contributes 
a Y chromosome, the result will be an 
XY pairing—a boy.
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388 Chapter 
12 
Development
important in the history of biology (International Human Genome Sequencing 
Consortium, 2003). 
The success of the Human Genome Project started a revolution in health care 
because scientists can identify the particular genes responsible for genetically caused 
disorders. It is already leading not only to the identifi cation of risk factors in chil-
dren, but also to the development of new treatments for physical and psychological 
disorders. 
For example, in gene therapy, health-care providers inject genes to correct par-
ticular diseases directly into a patient’s bloodstream. When the gene arrives at the 
location of a problem (or potential problem), it leads the body to produce chemicals 
that can alleviate the danger. In other cases, additional genes are inserted that replace 
missing or defective cells. Eventually, it also may be possible to “harvest” defective 
cells from a child prior to birth. These cells could be treated by gene therapy and 
reintroduced into the unborn child to repair the defect (Lymberis et al., 2004; Sato, 
Shimamura, & Takeuchi, 2007; Naldini, 2009). 
Although the promise of gene therapy is real, the number of diseases that can 
be treated today is fairly limited. Furthermore, the long-term success of gene ther-
apy remains unknown. In fact, after they initially seem to be cured, some recipients 
of gene therapy have relapsed, and some have suffered from unpleasant side 
effects. Still, the potential uses of gene therapy are growing rapidly. For example, 
such disorders as AIDS, cancer, rheumatoid arthritis, and macular degeneration are 
strong candidates for the procedure (Feret et al., 2007; Rossi, June, & Kohn, 2007; 
Miller et al., 2008).
The Earliest Development
When an egg becomes fertilized by the sperm, the resulting one-celled entity, called 
a  zygote,  immediately begins to develop. The zygote starts out as a microscopic 
speck. Three days after fertilization, though, the zygote increases to around 32 cells; 
within a week it has grown to 100–150 cells. These fi rst two weeks are known as the 
germinal period .
Two weeks after conception, the developing individual enters the embryonic 
period, which lasts from week 2 through week 8; he or she is now called an  embryo. 
As an embryo develops through an intricate, preprogrammed process of cell divi-
sion, it grows 10,000 times larger by 4 weeks of age and attains a length of about 
one-fi fth of an inch. At this point it has developed a rudimentary beating heart, a 
brain, an intestinal tract, and a number of other organs. Although all these organs 
are at a primitive stage of development, they are clearly recognizable. Moreover, 
by week 8, the embryo is about an inch long and has discernible arms, legs, and 
a face.
From week 8 and continuing until birth, the developing individual enters the 
fetal period and is called a  fetus.  At the start of this period, it begins to respond to 
touch; it bends its fi ngers when touched on the hand. At 16 to 18 weeks, its move-
ments become strong enough for the mother to sense them. At the same time, hair 
may begin to grow on its head, and the facial features become similar to those the 
child will display at birth. The major organs begin functioning, although the fetus 
could not be kept alive outside the mother. In addition, a lifetime’s worth of brain 
neurons are produced—although it is unclear whether the brain is capable of think-
ing at this early stage.

By week 24, a fetus has many of the characteristics it will display as a newborn. 

In fact, when an infant is born prematurely at this age, it can open and close its eyes; 
suck; cry; look up, down, and around; and even grasp objects placed in its hands. 
Within the womb the fetus continues to develop before birth. It begins to grow fatty 
deposits under the skin, and it gains weight. The fetus reaches the  age of viability,  the