How often have we heard comments such as “She

has her mother’s eyes” or “That nose is just like his
father’s”—because people cannot resist comparing a
newborn with his or her parents. Although a child may
not resemble either parent, there is a sound basis for
such comparisons, because the genetic makeup and
many of the traits of a child are the result of the chro-
mosomes inherited from mother and father.
In this chapter, we will cover some of the funda-
mentals of genetics and inheritance. First, however,
we will look at the development of a fertilized egg into
a functioning human being.

HUMAN DEVELOPMENT

During the 40 weeks of gestation, the embryo-fetus is
protected and nourished in the uterus of the mother.
A human being begins life as one cell, a fertilized egg
called a zygote, which develops into an individual
human being consisting of billions of cells organized
into the body systems with whose functions you are
now quite familiar.

FERTILIZATION

Although millions of sperm are deposited in the
vagina during sexual intercourse, only one will fertil-
ize an ovum. As the sperm swim through the fluid of
the uterus and fallopian tube, they undergo a final
metabolic change, called capacitation. This change
involves the acrosome, which becomes more fragile.
When sperm and egg make contact, the acrosomal
enzymes will digest the layers of cells and membrane
around an ovum.
Once a sperm nucleus enters the ovum, changes in
the egg cell membrane block the entry of other sperm.
The nucleus of the ovum completes the second mei-
otic division, and the nuclei of ovum and sperm fuse,
restoring the diploid number of chromosomes in the
zygote.
The human diploid number of 46 chromosomes is
actually 23 pairs of chromosomes; 23 from the sperm
and 23 from the egg. These 23 pairs consist of 22 pairs
of autosomes(designated by the numerals 1 through
22) and one pair of sex chromosomes. Women have
the sex chromosomes XX, and men have the sex chro-
mosomes XY. Figure 21–1 shows the inheritance of

gender. The Y chromosome has a gene that triggers

the development of male gonads in the embryo. In the

absence of the Y chromosome, the embryo will

develop as a female.

IMPLANTATION

Fertilization usually takes place within the fallopian

tube, and the zygote begins to divide even as it is being

swept toward the uterus. These are mitotic divisions

and are called cleavage. Refer to Fig. 21–2 as you read

the following.

The single-cell zygote divides into a two-cell stage,

four-cell stage, eight-cell stage, and so on. Three days

after fertilization there are 16 cells, which continue to

divide to form a solid sphere of cells called a morula

(see Box 21–1: Twins). As mitosis proceeds, this sphere

becomes hollow and is called a blastocyst, which is

still about the same size as the original zygote.

A fluid-filled blastocyst consists of an outer layer of

cells called the trophoblastand an inner cell mass that

contains the potential embryo. It is the blastocyst stage

that becomes implantedin the uterine wall, about 5 to

8 days after fertilization.

Successful implantation is not simply a matter of the

blastocyst contacting the uterine lining. The endo-

metrium produces carbohydrate “docking” molecules

toward the end of the uterine cycle, and the blastocyst

has a surface protein that fits the docking sites. Once

this “fit” is made, the trophoblast secretes enzymes to

digest the surface of the endometrium, creating a small

crater into which the blastocyst sinks. The trophoblast

will become the chorion, the embryonic membrane

that will form the fetal portion of the placenta.

Following implantation, the inner cell mass will grow

to become the embryo and other membranes.

The cells of the inner cell mass are embryonic stem

cells. In these cells, all of the DNA has the potential

for being switched on, that is, the potential exists for a

cell to develop into any of the 200 kinds of human

cells that will be present at birth. As the cells continue

to divide and become more numerous, some DNA

will be switched off in each cell, genes will become

inactive, and the possibilities for the specialization of

each cell will be narrowed down. The “switches” are

chemicals, few of which are known at present. A great

deal of research is focusing on discovering the stimuli

that turn a stem cell into a muscle cell rather than a

nerve cell or skin cell, and so on.

476 Human Development and Genetics