V

Bb

The probable results of crossing two rabbits that are heterozygous for black coat color are 50 percent heterozygous black individuals, 25 percent homozygous black individuals, and 25 percent homozygous brown individuals.

Example 3: Heterozygous X Heterozygous

In rabbits, the allele for black coat color (B) is dominant over the allele for brown coat color (b). The Punnett square in Figure 9-9 shows the predicted results of crossing two rabbits that are heterozygous (Bb) for coat color. As you can see, 1/4 (25 percent) of the offspring are predicted to have the genotype BB, 1/2 (50 percent) are predicted to have the genotype Bb, and 1/4 (25 percent) are predicted to have the genotype bb. Thus, 3/4 (75 percent) of the offspring resulting from this cross are predicted to have a black coat. One-fourth (25 percent) of the offspring are predicted to have a brown coat.

The ratio of the genotypes that appear in offspring is called the genotypic ratio. The probable genotypic ratio of the monohybrid cross represented in Figure 9-9 is 1 BB : 2 Bb : 1 bb. The ratio of the offspring's phenotypes is called the phenotypic ratio. The probable phenotypic ratio of the cross represented in Figure 9-9 is 3 black : 1 brown. b b

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