Red-green colour blindness is inherited as an X-linked recessive trait, with X^B representing the normal (dominant) allele and X^b representing the colour-blind (recessive) allele. What is the genotype of a female who displays the colour-blind phenotype?

A female inherits two X chromosomes, so both copies of the allele affect her phenotype. Because the colour-blind allele (X^b) is recessive, a female shows the colour-blind phenotype only when both her X chromosomes carry it, giving the genotype X^b X^b; a single X^B allele, as in the carrier genotype X^B X^b, is enough to produce normal colour vision.