The Testcross
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
A testcross crosses an unknown genotype, showing the dominant trait, against a homozygous recessive individual to reveal whether it is homozygous or heterozygous.
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A testcross producing only dominant offspring indicates a homozygous dominant parent; a roughly even split indicates heterozygous.
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A small testcross litter showing only the dominant trait does not rule out a heterozygous parent — larger samples resolve the ratio.