Sex-Linked Inheritance Help
Any gene located on the X chromosome or on the analogous Z chromosome in birds and other species is said to be sex-linked, or X-linked. The first sex-linked gene found in Drosophila was the recessive white-eye mutation. Reciprocal crosses involving autosomal traits yield similar results. Reciprocal crosses are carried out by mating a male with one phenotype (e.g., black fur coat) and a female with another phenotype (e.g., white fur coat) and then repeating the cross with a male and female of phenotypes that are opposites of the first cross (i.e., a white-coated male and a black-coated female). However, sex-linked traits do not show similar results in reciprocal crosses, as shown below. When white-eyed females are crossed with wild-type (red-eyed) males, all the male offspring have white eyes like their mother and all the female offspring have red eyes like their father.
This type of inheritance is due to the fact that the Y chromosome carries no alleles homologous to those at the white locus on the X chromosome. Whereas the X chromosome carries thousands of genes, the Y contains several hundreds of genes. Thus, males carry only one allele for sex-linked traits. This one-allelic condition is termed hemizygous in contrast to the homozygous or heterozygous possibilities in the female. If the F1 of Example 5.6 mate among themselves to produce an F2, a 1 red : 1 white phenotypic ratio is expected in both the males and females.
The reciprocal cross, where the sex-linked mutation appears in the male parent, results in the disappearance of the trait in the F1 and its reappearance only in the males of the F2. This type of skip generation inheritance also characterizes sex-linked genes.
Thus, a 3 red : 1 white phenotypic ratio is expected in the total F2 disregarding sex, but only the males show the mutant trait. The phenotypic ratio among the F2 males is 1 red : 1 white. All F2 females are phenotypically wild type.
Whenever working with problems involving sex linkage in this book, be sure to list the ratios for males and females separately unless specifically directed by the problem to do otherwise.
In normal diploid organisms with sex-determining mechanisms like those of humans or Drosophila, a trait governed by a sex-linked recessive gene usually manifests itself in the following manner: (1) it is usually found more frequently in the male than in the female of the species, (2) it fails to appear in females unless it also appeared in the paternal parent, (3) it seldom appears in both father and son, then only if the maternal parent is heterozygous. On the other hand, a trait governed by a sex-linked dominant gene usually manifests itself by (1) being found more frequently in the female than in the male of the species, (2) being found in all female offspring of a male that shows the trait, (3) failing to be transmitted to any son from a mother that did not exhibit the trait herself.
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