HUMAN BIOLOGY

XX XX

XY XY

XX

X

X

eggs sperm

diploid

germ cells

in female

meiosis, gamete

formation in both

female and male:

Fertilization:

sex chromosome

combinations

possible in new

individual

X

Y

X

Y

×

×



Y chromosome

present

appearance

of “uncommitted”

duct system of

embryo at

7 weeks

testes

Y chromosome

absent

ovaries

ovary

uterus

vagina

penis

testis

388 Chapter 20

the sex Chromosomes

A mother’s egg always carries an X chromosome, so

the father’s sperm determines a baby’s gender. If an

X-bearing sperm fertilizes an egg, the embryo will be XX

and develop into a female. On the other hand, if the sperm

has a Y chromosome, the embryo will be XY and develop

into a male (Figure 20.4A).

The genes on a Y chromosome include the master gene

for male sex determination, which has been dubbed SRY.

When the gene is expressed, testes form in the embryo

(Figure 20.4B). When that gene is “missing” because no

Y chromosome is present, ovaries form, and the developing

embryo is female.

Although the X chromosome has some genes associated

with sexual traits, such as the distribution of body fat, most

of its genes deal with nonsexual traits such as blood clot-

ting. Males have one X chromosome, so these genes can

be expressed in males as well as in females. The genes on

X and Y chromosomes are sometimes called X-linked

genes and Y-linked genes, respectively.

n Sex chromosomes carry genes associated with sexual traits.

n Links to Formation of eggs and sperm 16.2, 16.4

Gender is a question of X or y

It has been said that “men are from Mars, women are

from Venus.” While that topic goes far beyond biology

class, there are biological differences

between the sexes. You already

know that the X  chromosome is

the female sex chromosome and

the Y  chromosome is the male

sex chromosome. A female’s diploid

cells each have two X chromosomes,

so females are said to be XX. A

male’s diploid cells each have one

X chromosome and one Y, so males

are said to be XY. Each X chromo-

some carries an estimated 1,400 genes, but a Y chromosome

is much smaller and carries an estimated one hundred

genes, at most.

Despite their differences, the X and Y chromosomes

can be joined together briefly in a small region along their

length. This allows the X and Y to function as homologues

during meiosis.

A B

Eyewire, Inc./Getty Images

PhotoDisc/Getty Images

1.120.3

Figure 20.4 Animated! The father’s sperm determines whether

a baby will be male or female. A Males transmit their Y chromosome

to sons but not daughters. Males get their X chromosome from their

mother. B How the duct system in an early embryo develops into a

male or a female reproductive system. (© Cengage Learning)

X chromosome The female
sex chromosome.

X-linked gene Any
of the genes on an X
chromosome.

Y chromosome The male
sex chromosome.

Y-linked gene Any of the
genes on a Y chromosome.

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