A woman who is heterozygous for the sex-linked condition of red-green color blindness marries a man with normal vision. Which of these would you not expect to observe among their possible children?

Question

a.) A son who is color blind and a daughter with normal vision

b.) A son and a daughter with normal vision

c.) A son with normal vision and a daughter who is color blind

d.) Two daughters with normal vision

Answer 1

c.) A son with normal vision and a daughter who is color blind.



Color blindness is a sex-linked condition, meaning it is caused by a gene located on the X chromosome. Women have two X chromosomes, while men have one X and one Y chromosome. If a woman is heterozygous for the gene for color blindness (i.e. she has one dominant allele for normal vision and one recessive allele for color blindness), she has a 50% chance of passing on the allele for color blindness to her children. However, since only males have a Y chromosome, they can only inherit the X chromosome carrying the allele for color blindness from their mother. Therefore, a son born to a woman who is heterozygous for the gene for color blindness and a man with normal vision will not be color blind.