recessive genetic traits?

Question

In humans, red-green colorblindness is a sex-linked recessive trait. If man is colorblind, which of the following must be true?


A.His father is colorblind.

B.His mother is colorblind.

C. Both his parents are colorblind.

D. His father carries at least one colorblind allele.

E. His mother carries at least one colorblind allele.

Answer 1

E

Recessive sex-linked traits are carried on the X-chromosome, and are only expressed when the recessive X-chromosome gene is present and no dominant X-chromosome gene is present. This can occur in one of two ways: the individual is male, so the X-chromosome with the recessive gene is the only X-chromosome present; or the individual is female, but both X-chromosomes carry the same recessive gene.

A man's X-chromosome comes from his mother. So while the father of a colorblind man could certainly be colorblind, the younger man didn't inherit it from his father. This excludes A, C, and D. And while the mother could be colorblind, she doesn't have to be; she can carry either one or two recessive X-chromosome genes. So E is correct while B is too strong a condition.

Answer 2

E - the trait is on the X sex chromosome, so it came from mom. She's either colorblind (homozygous for the defect) or a carrier

Answer 3

E) Color blindness is a X chromosome trait, and for a boy to have it he must have received the recessive gene from his mother, since his father provided a Y chromosome.