test cross

When an individual shows a dominant trait, you cannot tell from looking whether it carries two dominant alleles or just one — both versions look the same. A test cross is the breeder's trick for finding out: mate the mystery individual with a partner that is homozygous recessive, and let the offspring give the answer.

The recessive partner can only contribute recessive alleles, so the offspring's appearance is decided entirely by what the unknown parent passes on. If that parent is homozygous dominant, every offspring shows the dominant trait. If it is heterozygous, about half the offspring show the recessive trait, exposing the hidden allele.

The test cross was a workhorse of classical genetics, used not only to read genotypes but also to measure recombination between linked genes, since the proportion of new allele combinations among offspring reflects how often crossing over occurs. With molecular tools we can now read many genotypes directly, but the logic of using a known partner to reveal an unknown one remains central.