X

► When a homozygous red-eyed female was crossed with a (hemizygous) white-eyed male, all the sons and daughters had red eyes, because red is dominant over white and all the progeny had inherited a wild-type X chromosome from their mothers (Figure 10.23a).

► However, in the reciprocal cross, in which a white-eyed female was mated with a red-eyed male, all the sons were white-eyed and all the daughters were red-eyed (Figure 10.23b).

► The sons from the reciprocal cross inherited their only X chromosome from their white-eyed mother; the Y chromosome they inherited from their father does not carry the eye color locus (Figure 10.23b).

► The daughters, on the other hand, got an X chromosome bearing the white allele from their mother and an X chromosome bearing the red allele from their father; they were therefore red-eyed heterozygotes (Figure 10.23b).

► When heterozygous females were mated with red-eyed males, half their sons had white eyes, but all their daughters had red eyes.

Together, these results showed that eye color was carried on the X chromosome and not on the Y.

Humans display many sex-linked characters

The human X chromosome carries about two thousand genes. The alleles at these loci follow the same pattern of inheritance as those for white eyes in Drosophila. One human X chromosome gene, for example, has a mutant recessive allele that leads to red-green color blindness, a hereditary disorder. Red-green color blindness appears in individuals who are ho-mozygous or hemizygous for the mutant allele.

Female who carries gene for phenotype of interest on one X chromosome

This woman carries the mutant allele but she is a phenotypically normal heterozygote.

Pedigree analysis of X-linked recessive phenotypes (Figure 10.24) reveals the following patterns:

► The phenotype appears much more often in males than in females, because only one copy of the rare allele is needed for its expression in males, while two copies must be present in females.

► A male with the mutation can pass it on only to his daughters; all his sons get his Y chromosome.

► Daughters who receive one mutant X chromosome are heterozygous carriers. They are phenotypically normal, but they can pass the mutant X to both sons and daughters (but do so only half of the time, on average, since half of their X chromosomes carry the normal allele).

► The mutant phenotype can skip a generation if the mutation passes from a male to his daughter (who will be phenotypically normal) and thus to her son.

Hemophilia A, which affected the family described at the beginning of this chapter, is an X-linked recessive phenotype, as are several other important human diseases, as we will see in later chapters. Human mutations inherited as X-linked dominant phenotypes are rarer than X-linked recessives because dominant phenotypes appear in every generation, and because people carrying the harmful mutation, even as heterozygotes, often fail to survive and reproduce. (Look at the four points above and try to determine what would happen if the mutation were dominant.)

The small human Y chromosome carries several dozen genes. Among them is the maleness determinant, SRY. Interestingly, for some genes on the Y, there are similar, but not identical, genes on the X. For example, one of the proteins

10.24 Red-Green Color Blindness is a Sex-Linked Trait in Humans

The mutant allele for red-green color blindness is inherited as an X-linked recessive.

This woman inherited the mutant X from her mother and a normal X from her father.

Female who carries gene for phenotype of interest on one X chromosome

This woman carries the mutant allele but she is a phenotypically normal heterozygote.

This woman inherited the mutant X from her mother and a normal X from her father.

Linked Recessive Carrier Father

This man inherited the mutant X chromosome from his mother and a normal Y from his father, and expresses the mutation. He passed his mutant X chromosome to his daughter, and she passed it on to her son.

Two siblings inherited the mutant X from their mother. The son expresses the mutation; his sister is a carrier.

In this test for red-green color blindness, people with normal color vision will see the number 15.

This man inherited the mutant X chromosome from his mother and a normal Y from his father, and expresses the mutation. He passed his mutant X chromosome to his daughter, and she passed it on to her son.

Two siblings inherited the mutant X from their mother. The son expresses the mutation; his sister is a carrier.

In this test for red-green color blindness, people with normal color vision will see the number 15.

that make up ribosomes has a gene on the Y that is expressed only in male cells, while the X-linked counterpart is expressed in both sexes. This means that there are "male" and "female" ribosomes; the significance of this phenomenon is unknown. Y-linked alleles are passed only from father to son. (You can verify this with a Punnett square.)

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