Red-green colour blindness is a condition caused by a recessive allele. The gene is carried on the X chromosome. A man is red-green colour blind, which of his relatives must carry the recessive allele?

Answer:

25) D

The man has chromosomes X and Y. He must have received the Y chromosome from his father and the X chromosome from is mother.
Since the condition is only carried on the X chromosome, his mother must carry the recessive allele.

His daughter must have the chromosomes X and X. The daughter must have received one X chromosome from her father (the man in the question) and one from her mother. Therefore, she must also carry the recessive allele.

So D is correct.