928
UNIT SIX
A daughter can inherit an X-linked recessive disorder
or trait if her father is affected and her mother is a carrier.
She inherits one affected X chromosome from each parent.
Without a biochemical test, though, a woman would not
know that she is a carrier of an X-linked recessive trait unless
she has an affected son.
For X-linked recessive traits that seriously impair
health, affected males may not feel well enough to have
children. A female affected by an X-linked trait must inherit
the mutant allele from a carrier mother and an affected
father, so such traits that are nearly as common among
females as males tend to be those associated with milder
phenotypes. Colorblindness is a mild X-linked trait—men
who are colorblind are as likely to have children as men
with full color vision.
Dominant disease-causing alleles on the X chromo-
some are rarely seen. Males are usually much more severely
affected than females, who have a second X to offer a protec-
tive effect. In a condition called incontinentia pigmenti, for
example, an affected girl has swirls of pigment in her skin
where melanin in the epidermis extends into the dermis. She
may have abnormal teeth, sparse hair, visual problems, and
seizures. However, males inheriting the dominant gene on
their X chromosomes are so severely affected that they do
not survive to be born.
Gender Effects on Phenotype
Certain autosomal traits are expressed differently in males
and females, due to differences between the sexes.
A
sex-limited trait
affects a structure or function present
in only males or only females. Such a gene may be X-linked
or autosomal. Beard growth and breast size are sex-limited
traits. A woman cannot grow a beard because she does not
manufacture sufficient hormones required for facial hair
growth, but she can pass to her sons the genes that specify
heavy beard growth. In animal breeding, milk yield and horn
development are important sex-limited traits.
In
sex-infl
uenced inheritance,
an allele is dominant in
one sex but recessive in the other. Again, such a gene may be
X-linked or autosomal. This difference in expression refl ects
hormonal differences between the sexes. For example, a gene
for hair growth pattern has two alleles, one that produces
hair all over the head and another that causes pattern bald-
ness. The baldness allele is dominant in males but recessive
in females, which is why more men than women are bald. A
heterozygous male is bald, but a heterozygous female is not.
A bald woman would have two mutant alleles.
About 1% of human genes exhibit
genomic imprinting,
in which the expression of a disorder differs depending upon
which parent transmits the disease-causing gene or chromo-
some. The phenotype may differ in degree of severity, in age
of onset, or even in the nature of the symptoms. The physical
basis of genomic imprinting is that methyl (—CH
3
) groups
are placed on the gene inherited from one parent, preventing
it from being transcribed and translated.
activities as bone growth, signal transduction, the synthesis of
hormones and receptors, and energy metabolism. The mem-
bers of the second functional group of Y chromosome genes
are similar in DNA sequence to certain genes on the X chro-
mosome, but they are not identical. These genes are expressed
in nearly all tissues, including those found only in males. The
third group of genes includes those unique to the Y chromo-
some. Many of them control male fertility, such as the
SRY
gene. Some cases of male infertility arise from tiny deletions
of these parts of the Y chromosome. Other genes in this group
encode proteins that participate in cell cycle control; proteins
that regulate gene expression; enzymes; and protein receptors
for immune system biochemicals.
Y-linked genes are transmitted only from fathers to sons,
because only males have Y chromosomes. The differences in
inheritance patterns of X-linked genes between females and
males result from the fact that any gene on the X chromosome
of a male is expressed in his phenotype, because he has no
second allele on a second X chromosome to mask its expres-
sion. The human male is
hemizygous
for X-linked traits
because he has only one copy of each X chromosome gene.
Red-green colorblindness
and the most common form of the
clotting disorder
hemophilia
are recessive X-linked traits.
A male always inherits his Y chromosome from his
father and his X chromosome from his mother. A female
inherits one X chromosome from each parent. If a mother is
heterozygous for a particular X-linked gene, then her son has
a 50% chance of inheriting either allele from her. X-linked
genes are therefore passed from mother to son. A male does
not receive an X chromosome from his father (he inherits the
Y chromosome from his father), so an X-linked trait is not
passed from father to son.
Consider the inheritance of hemophilia A. It is passed
from carrier mother to affected son with a risk of 50%,
because he can inherit either her normal allele or the mutant
one. A daughter has a 50% chance of inheriting the hemo-
philia allele and being a carrier like her mother and a 50%
chance of not inheriting the allele.
The contribution of the X chromosome is equal in males and females,
even though males have one X and females have two, because one
X is silenced in every somatic cell of a female mammal. A female is a
mosaic, with genes from her father’s X chromosome expressed in some
cells, and genes from her mother’s in others. This X inactivation occurs
at random and is detectable for some genes. A woman who is a carrier
(a heterozygote) for
Duchenne muscular dystrophy,
for example, has a
wild type allele for the dystrophin gene on one X chromosome and a
mutant allele on the other. Cells in which the X chromosome bearing
the wild type allele is inactivated do not produce the dystrophin pro-
tein. However, cells in which the mutant allele is inactivated produce
dystrophin. When a stain for dystrophin is applied to a sample of her
muscle tissue, only some cells may turn blue, revealing her carrier sta-
tus. If by chance many wild type dystrophin alleles are turned oF
in her
muscle cells, she may experience mild muscle weakness.
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