Page 130 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

122 Chapter 4 The Chromosome Theory of Inheritance

later chapters. It is nonetheless important to realize that Figure 4.26 Male pattern baldness, a sex-influenced trait.
other organisms compensate for sex chromosome differ- (a) John Adams (1735–1826), second president of the United
ences in alternative ways. Fruit flies, for example, hyperac- States, at about age 60. (b) John Quincy Adams (1767–1848), son
tivate the single X chromosome in XY (male) cells, so that of John Adams and the sixth president of the United States, at
about the same age. The father-to-son transmission suggests that
most X chromosome genes produce twice as much protein male pattern baldness in the Adams family is likely determined by
product as each X chromosome in a female. The nematode an allele of an autosomal gene.
C. elegans, in contrast, ratchets down the level of gene ac- a: © Bettmann/Corbis; b: © The Corcoran Gallery of Art/Corbis
tivity on each of the X chromosomes in XX hermaphro-
dites relative to the single X in XO males.

Maleness and Male Fertility Are the Only
Known Y-Linked Traits in Humans

Theoretically, phenotypes caused by mutations on the Y
chromosome should also be identifiable by pedigree analy-
sis. Such traits would pass from an affected father to all of
his sons, and from them to all future male descendants.
Females would neither exhibit nor transmit a Y-linked phe-
notype (see Table 4.5). However, besides the determination (a) (b)
of maleness itself, as well as contributions to sperm forma-
tion and thus male fertility, no clear-cut Y-linked visible
traits have turned up in humans. The paucity of known Y-
linked traits reflects the fact that, as mentioned earlier, the condition in which hair is lost prematurely from the top
small Y chromosome contains very few genes. Indeed, one of the head but not from the sides (Fig. 4.26), is a sex-
would expect the Y chromosome to have only a limited ef- influenced trait in humans. Although pattern baldness is a
fect on phenotype because normal XX females do perfectly complex trait that can be affected by many genes, an auto-
well without it. somal gene appears to play an important role in certain
families. Men in these families who are heterozygous for
the balding allele lose their hair while still in their 20s,
Autosomal Genes Contribute whereas heterozygous women do not show any signifi-
to Sexual Dimorphism cant hair loss. In contrast, homozygotes in both sexes be-
come bald (though the onset of baldness in homozygous
Not all genes that produce sexual dimorphism (differences women is usually much later in life than in homozygous
in the two sexes) reside on the X or Y chromosomes. Some men). This sex-influenced trait is thus dominant in men,
autosomal genes govern traits that appear in one sex but recessive in women.
not the other, or traits that are expressed differently in the
two sexes.
Sex-limited traits affect a structure or process that Mutations in Sex Determination
is found in one sex but not the other. Mutations in genes Pathway Genes Can Result in
for sex-limited traits can influence only the phenotype of Intersexuality Disorders
the sex that expresses those structures or processes. A
vivid example of a sex-limited trait occurs in Drosophila We previously saw that the SRY gene on the Y chromosome
males homozygous for an autosomal recessive mutation is essential to maleness because it initiates testis develop-
known as stuck, which affects the ability of mutant males ment early in embryogenesis. But the functions of many
to retract their penis and release the claspers by which genes are required for testis development, or for subsequent
they hold on to female genitalia during copulation. The events that rely on hormones made in the testes for the de-
mutant males have difficulty separating from females af- velopment of sexual organs. Some of these genes are auto-
ter mating. In extreme cases, both individuals die, for- somal and some are X-linked; in either case, an XY
ever caught in their embrace. Because females lack individual with mutant alleles for any of these genes may
penises and claspers, homozygous stuck mutant females have unusual intersexual phenotypes.
can mate normally. In one important example, XY people with nonfunc-
Sex-influenced traits show up in both sexes, but the tional mutant alleles of the X-linked AR gene specifying
expression of such traits may differ between the two sexes the androgen receptor have a disorder known as com-
because of hormonal differences. Pattern baldness, a plete androgen insensitivity syndrome (CAIS). These