A few more non-Mendelian situations… For some traits, sex matters

Sex Linked Traits Traits located on sex chromosomes Usually caused by recessive genes Appear more commonly in males (only one X) – If recessive, females must carry trait on both XXs – Females with one copy are carriers Males receive all X-linked traits from their mothers Ex. Color blindness & hemophilia (abnormal blood clotting)

Sex-Limited Traits Generally found on autosomes Only expressed in phenotype of one gender Ex. Barred coloring in chickens normally is visible only in the roosters; Milk production in humans (females only)

Sex-Influenced Traits Traits located on autosomes Genes are expressed differently for XX vs. XY Ex. Early balding (influenced by testosterone) – Heterozygous men (Bb) lose their hair; Bb women do not. – Homozygous men or women (bb) become bald. – The trait is dominant in men, recessive in women. Ex. Singing voice – same allele combo produces a high soprano female and low bass male

Practice Problems Cross a carrier female (for hemophilia) with a normal male. Of all their offspring, what is the probability they will produce a hemophiliac son? (H = normal blood, h = hemophilia) X H X h X H X H X H X H X h Y X H Y X h Y