Symbols for XLinked Genes

There are several different ways to record genotypes for X-linked traits. Sometimes the genotypes are recorded in the same fashion as for autosomal characteristics — the hemizygous males are simply given a single allele: the genotype of a female Drosophila with white eyes would be ww, and the genotype of a white-eyed hemizygous male would be w. Another method is to include the Y chromosome, designating it with a diagonal slash (/). With this method, the white-eyed female's genotype would still be ww and the white-eyed male's genotype would be w/. Perhaps the most useful method is to write the X and Y chromosomes in the genotype, designating the X-linked alleles with superscripts, as we have done in this chapter. With this method, a white-eyed female would be XwXw and a white-eyed male XwY. Using Xs and Ys in the genotype has the advantage of reminding us that the genes are X linked and that the male must always have a single allele, inherited from the mother.

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