Assume that short hair (L) is dominant to long hair (l) and black hair (B) is dominant to brown (b). If you find a black, short-haired animal, you can determine its genotype by crossing it to an animal with a genotype of

You can determine its genotype by crossing it to an animal with a recessive genotype, llbb.

Explanation:

A test cross is performed when there is an individual with an unknown genotype and we desire to know if it is a heterozygote individual or a dominant homozygote individual.

The cross occurs between the individual of unkown genotype and a recessive homozygote individual.