Biology / Unit 5 Bio 1037 short lesson, about 20 seconds
The Testcross
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The idea
A testcross crosses an unknown genotype, showing the dominant trait, against a homozygous recessive individual to reveal whether it is homozygous or heterozygous.
In practice
A testcross producing only dominant offspring indicates a homozygous dominant parent; a roughly even split indicates heterozygous.
Where people slip
A small testcross litter showing only the dominant trait does not rule out a heterozygous parent — larger samples resolve the ratio.
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