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

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In a testcross, the individual with unknown genotype is crossed with a homozygous recessive individual. This is because if the unknown individual is homozygous dominant, all the offspring should show the dominant phenotype, whereas if the unknown individual is heterozygous, only half of the offspring should show the recessive phenotype. However, if the unknown individual is homozygous recessive, all the offspring should show the recessive phenotype. Therefore, a homozygous recessive individual is the best choice for a testcross.

A Testcross Is a Mating of an Individual with an Unknown