Gene Interaction That Produces Novel Phenotypes

Let's first examine 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. This plant produces peppers in one of four colors: red, brown, yellow, or green. If a homozygous plant with red peppers is crossed with a homozygous plant with green peppers, all the F1 plants have red peppers ( FIGURE 5.6a). When the F1 are crossed with one another, the F2 are in a ratio of 9 red : 3 brown : 3 yellow : 1 green ( FIGURE 5.6b). This dihybrid ratio (Chapter 3) is produced by a cross between two plants that are both heterozygous for two loci (RrCc X RrCc). In peppers, a dominant allele R at the first locus produces a red pigment; the recessive allele r at this locus produces no red pigment. A dominant allele C at the second locus causes decomposition of the green pigment chlorophyll; the recessive allele c allows chlorophyll to persist. The genes at the two loci then interact to produce the colors seen in F2 peppers:

Genotype

Phenotype red brown yellow green

P generation

Red t

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