Sex Chromosomes
Of the 23 pairs of chromosomes in human somatic cells, 22 pairs
are called autosomes. are chromosomes that are not
directly involved in determining the sex (gender) of an individual.
The , one of the 23 pairs of chromosomes in
humans, contain genes that will determine the sex of the individual.
In humans and many other organisms, the two sex chromosomes
are referred to as the X and Y chromosomes. The genes that cause a
fertilized egg to develop into a male are located on the Y chromo-
some. Thus, any individual with a Y chromosome is male, and any
individual without a Y chromosome is female. For example, in
human males, the sex chromosomes are made up of one X chromo-
some and one Y chromosome (XY). The sex chromosomes in human
females consist of two X chromosomes (XX). Because a female can
donate only an X chromosome to her offspring, the sex of an off-
spring is determined by the male, who can donate either an X or a Y.
The structure and number of sex chromosomes vary in different
organisms. In some insects, such as grasshoppers, there is no Y
chromosome—the females are characterized as XX and the males
are characterized as XO (the O indicates the absence of a chromo-
some). In birds, moths, and butterflies, the male has two X chro-
mosomes and the female has only one.
Change in Chromosome Number
Each of an individual’s 46 chromosomes has thousands of genes.
Because genes play an important role in determining how a person’s
body develops and functions, the presence of all 46 chromosomes
is essential for normal develop-
ment and function. A person must
have the characteristic number of
chromosomes in his or her cells.
Humans who are missing even one
of the 46 chromosomes do not sur-
vive. Humans with more than two
copies of a chromosome, a condition
called trisomy (TRY soh mee), will
not develop properly. Abnormalities
in chromosome number can be
detected by analyzing a
(KAR ee uh tiep), a photo of the chro-
mosomes in a dividing cell that
shows the chromosomes arranged by
size. Figure 5shows a typical karyo-
type. A portion of a karyotype from
an individual with an extra copy
of chromosome 21 is also shown in
Figure 5. This condition is called
Down syndrome, or trisomy 21.
Short stature, a round face with
karyotype
sex chromosomes
Autosomes
122 CHAPTER 6Chromosomes and Cell Reproduction
Karyotypes are used to examine an individual’s chromosomes.
Figure 5 A human karyotype
To prepare a karyotype,
photographs of the
chromosomes are cut out,
arranged in pairs from largest
to smallest, and numbered.
People with Down syndrome have
three copies of chromosome 21 in
their karyotype.
The word chromosome
is from the Greek chroma,
meaning “color,” and
soma,meaning “body.”
Chromosomes were so
named because they
absorbed a colored dye
that made them more
visible under a microscope.
