Sex Chromosomes and Their Genes

The X and Y chromosomes carry genes, but they are inherited in different patterns than are autosomal genes because of the different sex chromosome constitutions in males and females. Traits transmitted on the X chromosome are said to be X-linked, and on the Y, Y-linked. The X chromosome has more than one thousand genes; the Y chromosome has only a few dozen 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 en code a variety of proteins that function in both sexes, participating in or controlling such activities as bone growth, signal transduction, the synthesis of hormones and receptors, and energy metabolism. The members of the second functional group of Y chromosome genes are very similar in DNA sequence to certain genes on the X chromosome, 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 chromosome. Many of them control male fertility, such as the SRY gene. Some cases of male infertility can be traced to 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. 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

X-bearing sperm cell


XX female

Y-bearing sperm cell


XY male

Figure 24.11

Sex determination. An egg contributes an X chromosome, and a sperm, either an X or a Y. If a Y-bearing sperm cell fertilizes an egg, the zygote is male (XY). If an X-bearing sperm cell fertilizes an egg, the zygote is female (XX). Sex is actually determined by a gene on the Y chromosome.

Sperm Blood Vessels

The X and Y chromosomes. The SRY gene, at the top of the short arm of the Y chromosome, starts the cascade of gene activity that directs development of a male (31,000x).

his phenotype, because he has no second allele on a second X chromosome to mask its expression. An allele on an X chromosome in a female may or may not be expressed depending upon whether it is dominant or recessive, and upon the nature of the allele on the second X chromosome. The human male is said to be hemizygous for X-linked traits because he has half the number of genes on the X chromosome than the female has. Red-green color blindness and the most common form of the clotting disorder hemophilia are examples of 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, her son has a 50% chance of inheriting either allele from her. X-linked genes are therefore passed from mother to son. Because a male does not receive an X chromosome from his father (he inherits the Y chromosome from his father), 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 hemophilia allele and being a carrier like her mother and a 50% chance of not carrying the allele.

To remedy the seeming inequity of cells of a female having two X chromosomes compared to the male's one, female mammalian embryos shut off one X chromosome in each somatic cell. Which of a female's X chromosomes is silenced—the one she inherited from her mother or the one from her father—occurs randomly. Therefore, a female is a mosaic, with her father's X chromosome expressed in some cells, and her mother's in others. This X inactivation is detectable for some heterozygous, X-linked 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 disease-causing allele on the other. Cells in which the X chromosome bearing the wild-type allele is inactivated do not produce the gene's protein product, dystrophin. 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, some cells turn blue, and others do not, revealing her carrier status. If by chance many of such a woman's wild-type dystrophin alleles are turned off in her muscle cells, she may experience mild muscle weakness and is called a manifesting heterozygote.

John Adams1807


Pattern baldness is a sex-influenced trait and was a genetic trademark of the illustrious Adams family. (a) John Adams (1735-1826) was the second president of the United States. He was the father of (b) John Quincy Adams (1767-1848), who was the sixth president. John Quincy was the father of (c) Charles Francis Adams (1807-1886), who was a diplomat and the father of (d) Henry Adams (1838-1918), who was an historian.

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. Because a female affected by an X-linked trait must inherit the mutant allele from a carrier mother and an affected father, such traits that are nearly as common among females as males tend to be those associated with milder phenotypes. Color blindness is a good example of 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 chromosome are extremely rare. Males are usually much more severely affected than females, who have a second X to offer a protective effect. In a condition called incontinentia pigmenti, for example, an affected girl has swirls of pigment on 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.

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