Zz

In these organisms, the female produces two types of gametes, carrying Z or W. Whether the egg is Z or W determines the sex of the offspring, in contrast to humans and fruit flies, in which the sperm, carrying either X or Y, determines the sex.

Genes on sex chromosomes are inherited in special ways

Genes on sex chromosomes do not show the Mendelian patterns of inheritance we have described above. In Drosophila and in humans, the Y chromosome carries few known genes, but a substantial number of genes affecting a great variety of characters are carried on the X chromosome. Any such gene is present in two copies in females, but in only one copy in males. Therefore, females may be heterozygous for genes that are on the X chromosome, but males will always be hemi-zygous for genes on the X chromosome—they will have only one copy of each, and it will be expressed. Thus, reciprocal crosses do not give identical results for characters whose genes are carried on the sex chromosomes, and these characters do not show the usual Mendelian ratios for the inheritance of genes located on autosomes.

The first and still one of the best examples of inheritance of characters governed by loci on the sex chromosomes (sex-linked inheritance) is that of eye color in Drosophila. The wild-type eye color of these flies is red. In 1910, Morgan discovered a mutation that causes white eyes. He experimented by crossing flies of the wild-type and mutant phenotypes. His results demonstrated that the eye color locus is on the X chromosome. Study Figure 10.23 as you follow the crosses and results:

10.23 Eye Color Is a Sex-Linked Trait in Drosophila Thomas Hunt Morgan demonstrated that a mutant allele that causes white eyes in Drosophila is carried on the X chromosome. Note that in this case, the reciprocal crosses do not have the same results.

Homozygous red-eyed female

, Wild-type allele

/Allele for white eyes /No allele at all

Hemizygous white-eyed male

Homozygous white-eyed female

Hemizygous red-eyed male

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