HUMAN BIOLOGY

Chromosomes and human GenetiCs 389

How do tHe features and functions of tHe

X and y seX cHromosomes compare?

• The X chromosome, the female sex chromosome, is much larger
  and carries more genes than the Y chromosome—the male sex
  chromosome.


• A person’s sex is determined by the father’s sperm, which can
  carry either an X chromosome or a Y chromosome. XY embryos
  develop as males, and XX embryos as females.


• In females, one of the two X chromosomes is inactivated soon
  after embryonic development begins.


• Sex chromosomes do not carry genes for sex-influenced traits,
  such as pattern baldness.


• Genes that govern secondary sexual characteristics are on the
  X chromosome. Male and female sex hormones determine which
  ones are expressed in a given individual.


• Sex-influenced traits are governed by genes on autosomes but
  are expressed differently in males and females.

taKe-home messaGe

some genes are expressed differently

in males and females

You may have noticed that many more men than women

have pattern baldness. This common form of hair loss in

men is an example of a

sex-influenced trait. Such

traits appear much more

often in one sex than in

the other, or else the phe-

notype differs depending

on whether the person is

male or female. Genes for

sex-influenced traits are

on autosomes, not on sex

chromosomes. The gene

allele that causes pattern

baldness acts like a domi-

nant gene in males but

behaves like a recessive gene in females. That is, a male

needs only to inherit one copy in order to become bald. A

woman will develop pattern baldness only if she has two

copies of the allele, and usually much

later in life than a male who has the

allele. The difference is due to dif-

ferences in the effects of male sex

hormones in females and males.

Secondary sexual characteristics

such as the growth of a man’s beard

and the development of a woman’s

breasts are governed by sex-limited

genes. Both males and females inherit the same genes (on

the X chromosome), but only the genes appropriate to a

person’s biological sex are turned on when a youngster

reaches puberty. Sex hormones once again are the “switch.”

in females, one X is inactivated

Since females have two X chromosomes and males have
only one, do females have twice as many X-linked genes
and therefore a double dose of their gene products?
Not really, because a compensating mechanism called
X  inactivation occurs in females. Apparently, most or
all of the genes on one of a female’s X chromosomes are
turned off soon after the first cleavages of the zygote.
In a given cell, either of the two X chromosomes can
be inactivated. The inactivated X is condensed into a
Barr body (Figure 20.5A). From then on, the same X chro-
mosome will be inactivated in all the descendants of the
cell. After X inactivation takes place, the embryo continues
to develop. Typically, a female’s body has patches of tis-
sue where the genes of the maternal X chromosome are
expressed, and other patches where the genes of the pater-
nal X chromosome are expressed.
Some females have a mutation that makes the mosaic
tissues visible (Figure 20.5C). For example, incontinentia
pigmenti is an X-linked disorder that affects the skin,
teeth, nails, and hair. In females who are heterozygous for
the trait (one X chromosome has the mutation and the other
X is normal), mosaic tissue shows up as lighter and darker
patches of skin.

Figure 20.5 Animated! X inactivation halts the functioning
of one of a female’s two X chromosomes. A Light micrographs
show Barr bodies (condensed X chromosomes) in the nuclei
of several cells. B The X chromosome in cells of males is not
condensed this way. C A “mosaic” tissue effect that shows up
in females who have incontinentia pigmenti. In darker patches
of this girl’s skin, the mutated X chromosome is active.

Rivera Collection/Superstock, Inc.

C

NMSB/Custom Medical Stock Photo

A B

William Strauss

Barr bodies

Barr body An X chromo-

some in which the genes

are inactivated.

X inactivation Silencing of

genes on one of the two X

chromosomes in a female’s

somatic cells.

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