Sex Linkage

Sex-linked traits are carried on the X or Y chromosome and therefore tend to be inherited by one sex more than the other. Men are more likely than women to have red-green color blindness or hemophilia, for example, because the allele for each is recessive and located on the X chromosome (X-linked). Women have two X chromosomes. If a woman inherits the recessive hemophilia allele (h) on one of her X chromosomes, there is still a good chance that her other X chromosome will carry a dominant allele (H). H codes for normal blood-clotting proteins, so her blood clots normally. Men, on the other hand, have only one X chromosome and normally express any recessive allele found there (fig. 4.18). Ironically, even though this hemophilia is far more common among men than women, a man can inherit it only from his mother. Why? Because only his mother contributes an X chromosome to him. If he inherits h on his mother's X chromosome, he will have hemophilia. He has no "sec-

Figure 4.18 Sex-linked Inheritance of Hemophilia. Left: A

female who inherits a recessive allele (h) for hemophilia from one parent may not exhibit the trait, because she is likely to inherit the dominant allele (H) for a normal blood-clotting protein from her other parent. Right: A male who inherits h from his mother will exhibit hemophilia, because the Y chromosome inherited from his father does not have a gene locus for the clotting protein, and therefore has no ability to mask the effect of h.

Figure 4.18 Sex-linked Inheritance of Hemophilia. Left: A

female who inherits a recessive allele (h) for hemophilia from one parent may not exhibit the trait, because she is likely to inherit the dominant allele (H) for a normal blood-clotting protein from her other parent. Right: A male who inherits h from his mother will exhibit hemophilia, because the Y chromosome inherited from his father does not have a gene locus for the clotting protein, and therefore has no ability to mask the effect of h.

ond chance" to inherit a normal allele on a second X chromosome. A woman, however, gets an X chromosome from both parents. Even if one parent transmits the recessive allele to her, the chances are high that she will inherit a normal allele from her other parent. She would have to have the extraordinarily bad luck to inherit it from both parents in order for her to have a trait such as hemophilia or red-green color blindness.

The X chromosome is thought to carry about 260 genes, most of which have nothing to do with determining an individual's sex. There are so few functional genes on the Y chromosome—concerned mainly with development of the testes—that virtually all sex-linked traits are associated with the X chromosome.

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