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)

XH Xh

XH XHXH XHXh

Y XHY XhY