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