Worked Problems

1. Seed weight in a particular plant species is determined by pairs of alleles at two loci (c+ c— and b+ b—) that are additive and equal in their effects. Plants with genotype c—c—b—b— have seeds that average 1 g in weight, whereas plants with genotype c+c+b+b+ have seeds that average 3.4 g in weight. A plant with genotype c—c—b—b— is crossed with a plant of genotype c+c+b+b+.

(a) What is the predicted weight of seeds from the progeny of this cross?

(b) If the Fj plants are intercrossed, what are the expected seed weights and proportions of the F2 plants?

The difference in average seed weight of the two parental genotypes is 3.4 g — 1 g = 2.4 g. These two genotypes differ in four genes; so, if the genes have equal and additive effects, each gene difference contributes an additional 24 g/4 = 6 g of weight to the 1-g weight of a plant with none of these contributing genes (c—c—b—b—).

The cross between the two homozygous genotypes produces the following Fj and F2 progeny:

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