A testcross is a mating of an individual with an unknown genotype and an individual that:

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The individual with the unknown genotype must be crossed with a homozygous recessive individual. This is because if the unknown individual is heterozygous, half of the offspring will have the dominant phenotype and half will have the recessive phenotype, making it difficult to determine the unknown genotype. However, if the unknown individual is crossed with a homozygous recessive individual, all of the offspring will have the dominant phenotype if the unknown individual is homozygous dominant or have a 1:1 ratio of dominant to recessive phenotype if the unknown individual is heterozygous. Therefore, a testcross with a homozygous recessive individual is the best choice to determine the genotype of an unknown individual.

A Testcross Is a Mating of an Individual with an Unknown