What is the function of a test cross in genetics?

A test cross is a breeding experiment designed specifically to determine the genotype of an individual exhibiting a dominant phenotype. When an organism shows a dominant trait, it can either be homozygous (having two identical alleles for that trait) or heterozygous (having one dominant allele and one recessive allele). By crossing this individual with another that is homozygous recessive for the trait in question, geneticists can observe the phenotypes of the offspring to infer the genotype of the dominant individual.

If all the offspring display the dominant phenotype, it indicates that the individual in question is homozygous dominant. Conversely, if there are offspring displaying the recessive phenotype, it shows that the individual is heterozygous. Thus, the primary function of a test cross is to clarify the genetic makeup of an organism that is phenotypically dominant by analyzing the resulting progeny.