Download Sex-Influenced Genes - NCEA Level 2 Biology

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.