Human Physiology, 14th edition (2016)

704 Chapter 20

the neutrophils in a female ( fig. 20.3 ). These can provide a con-

venient microscopic test for the chromosomal sex of a person.

Actually, depending on the individual woman, about 15% of

the genes in the inactive X chromosome escape inactivation and

another 10% are only partially inactivated.

Formation of Testes and Ovaries

Following conception, the gonads of males and females are sim-

ilar in appearance for the first 40 or so days of development.

During this time, cells that will give rise to sperm (called sper-

matogonia ) and cells that will give rise to ova (called oogonia )

migrate from the yolk sac to the developing embryonic gonads.

At this stage, the embryonic structures have the potential to

become either testes or ovaries. The hypothetical substance that

promotes their conversion to testes ( fig. 20.4 ) has been called the

testis-determining factor (TDF).

Although it has long been recognized that male sex is

determined by the presence of a Y chromosome, the genes

involved have only recently been localized. In females, the

Y chromosome must be absent for pathways directed by genes

in the X chromosomes to result in the development of ovaries.

However, in rare male babies with XX genotypes, scientists

discovered that one of the X chromosomes contains a segment

of the Y chromosome—the result of an error that occurred

however, all of the ova will normally contain one X chromo-
some. Since all ova contain one X chromosome, whereas some
sperm are X-bearing and others are Y-bearing, the chromo-
somal sex of the zygote is determined by the fertilizing sperm
cell. If a Y-bearing sperm cell fertilizes the ovum, the zygote
will be XY and male; if an X-bearing sperm cell fertilizes the
ovum, the zygote will be XX and female.
Each diploid cell in a woman inherits two X chromosomes,
but because of X chromosome inactivation, one is inactive
and only one is fully active. X chromosome inactivation, like
genomic imprinting, results in the expression of only one allele
of a pair. This helps to explain why many congenital disor-
ders involve X-linked genes. Whether the inactivated X chro-
mosome is the one inherited from the mother or the father is
completely random, so a woman’s cells are a mosaic in which
the active X chromosome may be derived from either parent.
The inactive X chromosome forms a clump of heterochromatin,
which can be seen as a dark spot, called a Barr body, in cheek
cells, and a “drumstick” appendage in the nucleus of some of

Figure 20.3 Barr bodies. The nuclei of cheek cells from
females ( a ) have Barr bodies ( arrow ). These are formed from one
of the X chromosomes, which is inactive. No Barr body is present
in a cell obtained from a male because males have only one X
chromosome, which remains active. Some neutrophils obtained
from females ( b ) have a “drumstick-like” appendage ( arrow ) that
is not found in the neutrophils of males.

Barr body

(a)

(b)

Figure 20.4 The chromosomal sex and the

development of embryonic gonads. The very early embryo

has “indifferent gonads” that can develop into either testes or

ovaries. The testis-determining factor (TDF) is a gene located on

the Y chromosome. In the absence of TDF, ovaries will develop.

Indifferent

gonads

X

Ovum Sperm

XX

Zygote

Y

X

Sperm Ovum

XY

Zygote

Testes Ovaries

TDF No TDF

(Follicles do not develop

until third trimester)

Seminiferous

tubules

Interstitial

cells

Develop in early

embryo

X X