What does a test cross help determine?

A test cross is a breeding experiment used to determine the genotype of an organism with a dominant phenotype. When an organism exhibits a dominant phenotype, it is not immediately clear whether it possesses a homozygous dominant genotype (having two copies of the dominant allele) or a heterozygous genotype (having one dominant and one recessive allele). By crossing this organism with another that is homozygous recessive (which expresses the recessive phenotype), the offspring produced can provide insight into the parent's genotype.

If all the offspring display the dominant phenotype, it suggests that the parent in question is likely homozygous dominant. However, if any offspring show the recessive phenotype, it indicates that the parent must be heterozygous. Thus, the test cross allows for determination of whether the dominant phenotype observed is due solely to a homozygous dominant genotype or a mix with a recessive allele.