testcross

testcross

Experiment in which an organism with unknown genetic makeup is mated with an organism whose entire genetic makeup for a trait is known, to determine which genes are carried by the former. In a breed of dog, for example, in which the gene for black coat colour is dominant over the gene for red coat colour (see dominance), a dog with a black coat may be pure (with two genes for black coat colour) or hybrid (with one gene for black and one for red). When its mating with a red-coated dog (which always has two genes for red coat colour only) produces only black offspring, the black-coated parent must be pure, whereas if some of the young are red, the parent must be hybrid.