How can a pure white mouse help determine the genotype of a black mouse? (Hint: We cannot see genotype but we can see differences in the phenotype of their offspring).

A white mouse is purely recessive, and will produce black offspring if the other mouse contributes a black gene.

Explanation:

If Bb then the offspring will be 50% Bb and 50% bb. This is called a test cross, where we use a recessive to determine the genotype of a dominant phenotype.