Boys can inherit the recessive allele (c) that causes red-green colour blindness from their mother, not from their father. The allele for normal red and green vision is C. Which of the following genotypes are possible in men?
A. c only
B. C or c only
C. CC or cc only
D. CC, Cc or cc only

B

The notation in this question was not the usual sex linked notation. It would have been difficult to write it in this way as this would have made the question too easy. Nevertheless, the question turned out to be a good discriminator while still not proving too difficult.