318 CHAPTER 8
Prenatal Development
From conception to the actual birth of the baby is a period of approximately 9 months,
during which a single cell becomes a complete infant. It is also during this time that many
things can have a positive or negative influence on the developing infant.Fertilization
8.4 Explain the process of fertilization, including the twinning process.
When an egg (also called an ovum) and a sperm unite in the process of fertilization,
the resulting single cell will have a total of 46 chromosomes and is called a zygote. Nor-
mally, the zygote will begin to divide, first into two cells, then four, then eight, and so on,
with each new cell also having 46 chromosomes, because the DNA molecules produce
duplicates, or copies, of themselves before each division. (This division process is called
mitosis.) Eventually, the mass of cells becomes a baby. Sometimes this division process
doesn’t work exactly this way, and twins or multiples are the result.
There are actually two kinds of twins (see Figure 8. 3 ). Twins who are commonly
referred to as “identical” are monozygotic twins, meaning that the two babies come from
one (mono) fertilized egg (zygote). Early in the division process, the mass of cells splits
completely—no one knows exactly why—into two separate masses, each of which will
develop into a separate infant. The infants will be the same sex and have identical fea-
tures because they each possess the same set of 46 chromosomes. The other type of twin
is more an accident of timing and is more common in women who are older and who are
from certain ethnic groups (Allen & Parisi, 1990; Bonnelykke, 1990; Imaizumi, 1998). A
woman’s body may either release more than one egg at a time or release an egg in a later
ovulation period after a woman has already conceived once. If two eggs are fertilized,
the woman may give birth to fraternal or dizygotic twins (two zygotes), or possibly trip-
lets or some other multiple number of babies (Bryan & Hallett, 2001). This is also more
likely to happen to women who are taking fertility drugs to help them get pregnant.
Pregnancies involving multiple babies are often very high risk and can be associ-
ated with premature birth and low birth weight, both factors in possible long-term dis-
abilities in both physical and cognitive areas. Some of the babies may not survive, or
doctors might actually recommend selective termination of some of the babies to increase
the chances of survival for the remaining infants (Qin et al., 2015; Wilkinson et al., 2015).
This is a concern of an area called bioethics, the study of ethical and moral issues brought
about by new advances in biology and medicine
and how those advances should influence policies
and practices (Muzur, 2014; Qin et al., 2015).
For developmental psychologists, twins
provide an important way to look at the contri-
bution of nature and nurture to human devel-
opment. Researchers may seek out genetically
identical twins who have been separated at birth,
looking at all the ways those twins are alike in
spite of being raised in different environments. It
should be noted that the environments in which
children are raised within a particular culture are
not necessarily that much different, so twin stud-
ies are not a perfect method. Researchers may
also compare children who are adopted to their
adoptive parents (an environmental influence)
and to their biological parents (the genetic influ-
ences). to Learning Objective 13.12.zygote
cell resulting from the uniting of the
ovum and sperm.
fertilization
the union of the ovum and sperm.
ovum
the female sex cell, or egg.
monozygotic twins
identical twins formed when one
zygote splits into two separate masses
of cells, each of which develops into a
separate embryo.
dizygotic twins
often called fraternal twins, occurring
when two individual eggs get fertilized
by separate sperm, resulting in two
zygotes in the uterus at the same time.
bioethics
the study of ethical and moral issues
brought about by new advances in
biology and medicine.
Figure 8.3 Monozygotic and Dizygotic Twins
Because identical twins come from one fertilized egg (zygote), they are called monozygotic.
Fraternal twins, who come from two different fertilized eggs, are called dizygotic.
(^12)
(^12)
If both are fertilized by separate
sperm, two fetuses form. Gene-
tically they are just ordinary siblings.
The egg splits into halves.
Each develops into a fetus with
the same genetic composition.
Accounting for about 1-in-250
births, these are created when a
single egg is fertilized by one sperm.
Twice as common as identicals,
fraternals arise when two eggs
are released at once.
Identical twins
Fraternal twins
Interactive