A normal couple (not suffering from colour blindness) has a colour blind child. What are the possible genotypes of the couple?
XY and XcX
Since the male is normal, the genotype has to be XY. The female can be either XX, XcX or XcXc. Since the female is normal, she has to be either XX or XcX. For the child to be colour blind, the mother has to be a carrier at least. Therefore, the genotype of this mother has to be XcX.