Sex-Influenced Genes

Sex-Influenced Genes Certain traits, although not located on the sex chromosomes, have different appearances in males and females. E.g. Pattern Baldness

Pattern Baldness This is inherited and controlled by a single gene. In females the gene acts as a recessive, so a woman must have two recessive genes to show baldness. In men, only one baldness gene is needed.

Pattern Baldness BB – full hair in both sexes Bb – baldness in men, not in women bb – baldness in both sexes Women tend to lose hair evenly all over the head resulting in very thin hair. This is more pronounced after menopause.

Sex-Influenced Genes These genes are influenced by the presence of sex hormones. E.g. a bull may carry genes for high milk production, but he will not give milk. He would be a good sire for a dairy herd. Secondary sexual characteristics depend on the presence of the sex hormones for their expression. These in turn depend on the presence of X and Y chromosomes.

Sex-Influenced Genes E.g. a single pair of alleles determines the presence or absence of horns on sheep. H = horns is dominant in males but recessive in females; h = hornless is recessive in males but dominant in females. HH- horns in both sexes Hh- horns in males hh- no horns in both sexes.