Question

A man and his wife both have normal colour vision. The woman gives birth to a colour blind daughter. The man claims that he could not have fathered this daughter. Does genetics back his claim?

Answer 1

Genetics supports his claim. Color blindness is a sex-linked recessive trait. A girl could develop it only if she received X chromosomes with the disease allele from both parents. Mother could be heterozygous and thus, have normal vision still carrying the other X chromosome with disease allele. However, if father donated X chromosome with disease allele, he should be suffering from color blindness himself, as men have only one X chromosome. As father is not suffering from color blindness, he is not the biological parent.