Gene Interaction That Produces Novel Phenotypes

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Let’s begin by examining gene interaction in which genes at two loci interact to produce a single characteristic. Fruit color in the pepper Capsicum annuum is determined in this way. Certain types of pepper plants produce fruits in one of four colors: red, peach, orange (sometimes called yellow), and cream (or white). If a homozygous plant with red peppers is crossed with a homozygous plant with cream peppers, all the F1 plants have red peppers (Figure 4.17a). When the F1 are crossed with each other, the F2 show a ratio of 9 red : 3 peach : 3 orange : 1 cream (Figure 4.17b). This dihybrid ratio (see Chapter 3) is produced by a cross between two plants that are both heterozygous for two loci (Y+y C+c × Y+y C+c). In this example, the Y locus and the C locus interact to produce a single phenotype—the color of the pepper:

Genotype Phenotype
Y+_ C+_ red
Y+_ cc peach
yy C+_ orange
yy cc cream
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Figure 4.17: Interaction between genes at two loci determines a single characteristic, fruit color, in the pepper Capsicum annuum.

To illustrate how Mendel’s rules of heredity can be used to understand the inheritance of characteristics determined by gene interaction, let’s consider a testcross between an F1 plant from the cross in Figure 4.17 (Y+y C+c) and a plant with cream peppers (yy cc). As outlined in Chapter 3 for independent loci, we can work this cross by breaking it down into two simple crosses. At the first locus, the heterozygote Y+y is crossed with the homozygote yy; this cross produces ½ Y+y and ½ yy progeny. Similarly, at the second locus, the heterozygous genotype C+c is crossed with the homozygous genotype cc, producing ½ C+c and ½ cc progeny. In accord with Mendel’s principle of independent assortment, these single-locus ratios can be combined by using the multiplication rule: the probability of obtaining the genotype Y+y C+c is the probability of Y+y (½) multiplied by the probability of C+c (½), or ½ × ½ = ¼. The probabilities of each progeny genotype resulting from the testcross are

Progeny Genotype Probability at each locus Overall probability Phenotype
Y+y C+c ½ × ½ = ¼ red peppers
Y+y cc ½ × ½ = ¼ peach peppers
yy C+c ½ × ½ = ¼ orange peppers
Yy cc ½ × ½ = ¼ cream peppers

When you work problems with gene interaction, it is especially important to determine the probabilities of single-locus genotypes and to multiply the probabilities of genotypes, not phenotypes, because the phenotypes cannot be determined without considering the effects of the genotypes at all the contributing loci. image TRY PROBLEM 33