Page 103 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.2 Sex Chromosomes and Sex Determination 95

If sex is an inherited trait determined by a pair of sex Figure 4.7 Sex reversal. Sex-reversed XX males have a part of
chromosomes that separate to different cells during gamete the Y including the SRY gene on one of their X chromosomes. Sex-
formation, then an XX × XY cross could account for both reversed XY females lack SRY on their Y chromosome either because
the mutual exclusion of sexes and the near 1:1 ratio of it has been replaced by part of the X chromosome or because it has
been inactivated by mutation.
males to females, which are hallmark features of sex deter-
mination (Fig. 4.6b). And if chromosomes carry informa- Normal Sex-reversed
tion defining the two contrasting sex phenotypes, we can +
easily infer that chromosomes also carry genetic informa- SRY
tion specifying other characteristics as well. SRY – SRY –
SRY +

In Humans, the SRY Gene
Determines Maleness

You have just seen that humans and other mammals have a
pair of sex chromosomes that are identical in the XX female
but different in the XY male. Several studies have shown that XX XY XX XY XY
in humans, it is the presence or absence of the Y that actually
makes the difference; that is, any person carrying a Y chro- (Fig. 4.8). The two PARs (PAR1 and PAR2) together con-
mosome will look like a male. For example, rare humans tain about 30 genes, copies of which are found on both the X
with two X and one Y chromosome (XXY) are males dis- and Y chromosomes.
playing certain abnormalities collectively called Klinefelter Most of the Y chromosome, however, is called the
syndrome. Klinefelter males are typically tall, thin, and ster- male-specific region (MSY) (Fig. 4.8); the functions of only
ile, and they sometimes show mental retardation. That these some of the genes in the MSY are understood. The MSY in-
individuals are males shows that two X chromosomes are cludes four Y-specific (and therefore male-specific) genes:
insufficient for female development in the presence of a Y. SRY and three genes required for spermatogenesis. The name
In contrast, humans carrying an X and no second sex MSY is somewhat misleading because eight of the genes in
chromosome (XO) are females with Turner syndrome. Turner the MSY also exist on the X chromosome, but unlike the PAR
females are usually sterile, lack secondary sexual characteris- genes, they are not grouped together in one region of either the
tics such as pubic hair, are of short stature, and have folds of X or Y. These eight MSY genes affect the functions of cells
skin between their necks and shoulders (webbed necks). Even and tissues all over the body. In fact, several of these MSY
though these individuals have only one X chromosome, they genes shared with X are essential for male viability because
develop as females because they have no Y chromosome. without the Y-linked copies, the single gene copies on the X
In 1990, researchers discovered that it is not the entire chromosome do not supply sufficient protein. (Females nor-
Y chromosome, but rather a single Y-chromosome-specific mally express both alleles of the X-linked copies of these eight
gene called SRY (sex determining region of Y) that is the genes, as these genes escape a phenomenon described later in
primary determinant of maleness. The evidence implicat- this chapter.)
ing SRY came from so-called sex reversal: the existence of
XX males and XY females (Fig. 4.7). In many sex-reversed Figure 4.8 Human sex chromosomes have both shared
XX males, one of the two X chromosomes carries a portion and unique genes. PAR1 and PAR2 (black) are homologous
regions of the X and Y chromosomes that together contain about
of the Y chromosome. Although in different XX males, 30 genes. The MSY region contains genes needed for maleness
different portions of the Y chromosome are found on the X, itself (SRY), genes for male fertility, and essential genes shared with
one particular gene—SRY—is always present. Sex-reversed the X required for male viability because their X-linked counterparts
XY females, in contrast, always have a Y chromosome alone do not produce enough protein.
lacking a functional SRY gene; the portion of the Y chro- PAR1 PAR1
mosome containing SRY is either replaced by a portion of
the X chromosome, or the Y contains a nonfunctional mu- SRY
tant copy of SRY (Fig. 4.7). Later experiments with mice MSY Includes:
• SRY
confirmed that SRY indeed determines maleness. These region • 3 Male fertility genes
• 8 Essential genes shared with X
experiments are described in the Fast Forward Box
Transgenic Mice Prove That SRY Is the Maleness Factor.
SRY is one of about 110 protein-coding genes on the Y PAR2
chromosome. The two ends of the Y chromosome are called PAR2
the pseudoautosomal regions (PARs) because homologous X Y
~ 110
~ 1100
DNA sequences are present at the ends of the X chromosome genes genes