Question

Color-blindness is an x-linked recessive condition. a man with normal vision is married to a woman with normal vision, but whose dad is color-blind. the couple has a child with turner syndrome, and who is also color-blind. where did the non-disjunction occur?

Answer 1

Since color blindness is a sex-linked recessive genetic disorder, it is carried by a X chromosome. Males (XY) are affected if they get the affected X from their mother, since males only have one X, and it is therefore always expressed. Females (XX), however, get one X from their mother and the other from their father. So both parents have to pass a color blind X to be affected (both alleles, by the definition of recessive).
Even though the mother is not affected, she's carrying it because her father was color blind. And the father is not affected. However, in Turner's Syndrome, one X is destroyed after fertilization, so the other X is the daughter's primary (only remaining) X. Note: Turners only affects females. Therefore, this is a rare situation in which a daughter gets affected by an X-linked recessive condition.