R Y

round, yellow

3/16

R_yy

round, green

3/16

rrY_

wrinkled, yellow

1/16

rryy

wrinkled, green

In this example, the genes showed two kinds of independence. First, the genes at each locus are independent in their assortment in meiosis, which is what produces the 9:3:3:1 ratio of phenotypes in the progeny, in accord with Mendel's principle of independent assortment. Second, the genes are independent in their phenotypic expression; the R and r alleles affect only the shape of the seed and have no influence on the color of the endosperm; the Y and y alleles affect only color and have no influence on the shape of the seed.

Frequently, genes exhibit independent assortment but do not act independently in their phenotypic expression; instead, the effects of genes at one locus depend on the presence of genes at other loci. This type of interaction between the effects of genes at different loci (genes that are not allelic) is termed gene interaction. With gene interaction, the products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone. In our consideration of gene interaction, we'll focus primarily on interaction between the effects of genes at two loci, although interactions among genes at three, four, or more loci are common.

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