927
CHAPTER TWENTY-FOUR
Genetics and Genomics
are different. This is not the case for all types of animals. In
birds, for example, the female is the heterogametic sex.
Sex Determination
Maleness derives from a
Y
chromosome gene called
SRY,
for
sex-determining region of the
Y.
The
SRY
gene encodes a
type of protein called a transcription factor, which switches
on other genes.
SRY
activates transcription of genes that
direct development of male structures in the embryo, while
suppressing formation of female structures.
Figure 24.11
shows the sex chromosomes. Absence of the
SRY
transcrip-
tion factor, plus expression of a gene called
Wnt4,
triggers
development of a female body.
Genes on the Sex Chromosomes
Genes that are part of the X and Y chromosomes are inher-
ited in different patterns than are autosomal genes because
of the different sex chromosome constitutions of males
and females. Traits transmitted on the X chromosome are
X-linked, and on the Y, Y-linked. The X chromosome has
more than 1,500 genes; the Y chromosome has only 231 pro-
tein-encoding genes.
Y-linked genes are considered in three groups, based on
their similarity to X-linked genes. One group consists of genes
at the tips of the Y chromosome that have counterparts on the
X chromosome. These genes encode a variety of proteins that
function in both sexes, participating in or controlling such
Studies on well-nourished populations indicate that about 90% of
variation in height is due to genetics. In populations where nutrition
is inadequate, environmental eF
ects on height—stunting—are more
pronounced.
PRACTICE
12
How does polygenic inheritance make possible many variations of
a trait?
13
How can two genes specify ±
ve phenotypes?
14
How may the environment in²
uence gene expression?
24.5
MATTERS OF SEX
Human somatic (nonsex) cells include an X and a Y chro-
mosome in males and two X chromosomes in females. All
eggs carry a single X chromosome, and sperm carry either
an X or a Y chromosome. Sex is determined at conception: a
Y-bearing sperm fertilizing an egg conceives a male, and an
X-bearing sperm conceives a female
(f g. 24.10)
. The female
is termed the homogametic sex because she has two of the
same type of sex chromosome, and the human male is called
the heterogametic sex because his two sex chromosomes
FIGURE 24.10
Sex determination. An egg contributes an X
chromosome, and a sperm, either an X or a Y. If an X-bearing sperm
fertilizes an egg, the zygote is female (XX). If a Y-bearing sperm
fertilizes an egg, the zygote is male (XY). A gene on the Y chromosome,
SRY,
determines sex.
±±
Y-bearing
sperm
X
Egg
X
Egg
XX
female
XY
male
Y
X-bearing
sperm
X
SRY gene
X chromosome
Y chromosome
FIGURE 24.11
The X and Y chromosomes. The
SRY
gene, at one
end of the short arm of the Y chromosome, starts the cascade of gene
activity that directs development of a male (31,000×).
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