A test cross is a genetic tool used to determine the genotype of an organism with a dominant phenotype but unknown genotype. It involves crossing the organism in question with an organism that is homozygous recessive for the trait of interest. The resulting offspring can then be examined to determine the genotype of the unknown parent. If the test cross results in any recessive offspring, then the parent organism is heterozygous for the allele in question. If the test cross results in only phenotypically dominant offspring, then the parent organism is homozygous dominant for the allele.

Test crosses can be used to determine whether a dominant phenotype is homozygous or heterozygous for a specific allele. They can also be used to determine if two genes are linked or unlinked by mating with a known heterozygote. Test crosses require large numbers of offspring to produce reliable data for meaningful conclusions. While test crosses were once a fundamental tool in genetics, they have become less commonly used with the advent of genetic screening and genome mapping.