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What is a testcross?

A testcross is the mating of an organism with an unknown genotype with an organism whose genetic makeup for a trait is known, to determine the alleles (versions of genes) controlling the trait carried by the former. An organism displaying the dominant phenotype is mated with a purebred (homozygous) recessive individual of the same species, and the phenotype of the progeny is checked to determine the genotype of the dominant parent.