B

Bb

Bb

Bb

Bb

Example 4: Testcross

Recall that in guinea pigs, both BB and Bb result in a black coat. How might you determine whether a black guinea pig is homozy-gous (BB) or heterozygous (Bb)? You could perform a testcross, in which an individual of unknown genotype is crossed with a homozygous recessive individual. A testcross can determine the genotype of any individual whose phenotype expresses the dominant trait. You can see from Figure 9-10 that if the black guinea pig of unknown genotype is homozygous black, all offspring will be black. If the individual with the unknown genotype is heterozygous black, about half of the offspring will be black. In reality, if the cross produced any brown offspring, the genotype of the black-coated parent is likely to be heterozygous.

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