Sex Influenced and Sex Limited Characteristics

Sex influenced characteristics are determined by autosomal genes and are inherited according to Mendel's principles, but they are expressed differently in males and females. In this case, a particular trait is more readily expressed in one sex; in other words, the trait has higher penetrance (see p. 000 in Chapter 3) in one of the sexes.

For example, the presence of a beard on some goats is determined by an autosomal gene (B) that is dominant in males and recessive in females. In males, a single allele is required for the expression of this trait: both the homozygote (BbBb) and the heterozygote (BB) have beards, whereas the B+B+ male is beardless. In contrast, females require two alleles in order for this trait to be expressed: the homozygote BB has a beard, whereas the heterozygote (BB) and the other homozygote (BB) are beardless. The key to understanding the expression of the bearded gene is to look at the heterozygote. In males (for which the presence of a beard is dominant), the heterozygous genotype produces a beard but, in females (for which the presence of a beard is recessive and its absence is dominant), the heterozygous genotype produces a goat without a beard.

FIGURE 5.11a illustrates a cross between a beardless male (B+B+) and a bearded female (BbBb). The alleles

P generation

Beardless $

Bearded 2

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