Sex-influenced characteristics: Beard in goats
Sex-influenced characteristics are traits that are encoded by genes located on the autosomes (non-sex
chromosomes) but are expressed differently depending on the sex of the individual. In other words,
these traits are influenced by the sex hormones present in the body.
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 in one of the
sexes.
For example, the presence of a beard on some goats is determined by an allele at an autosomal locus
(Bb) that is dominant in males and recessive in females. In males, a single beard allele is required for the
expression of this trait: both the homozygote (Bb Bb) and the heterozygote (Bb B+) have beards, whereas
the B+ B+ male is beardless. In contrast, females require two beard alleles in order for this trait to be
expressed: the homozygote Bb Bb has a beard, whereas the heterozygote (Bb B+) and the other
homozygote (B+B+) are beardless.
The key to understanding the expression of the beard allele 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 absence of a beard is dominant), the heterozygous genotype produces a goat
without a beard.
