SexInfluenced Genes SexInfluenced Genes Certain traits although not

Sex-Influenced Genes

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

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

Pattern Baldness BB – full hair in both sexes n Bb – baldness in men, not in women n bb – baldness in both sexes n n 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. n 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. n 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. n

Sex-Influenced Genes n 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.