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Modern Genetic Analysis Chapter 6. Gene Interaction
Solved Problem

1.

Beetles of a certain species may have green, blue, or turquoise wing covers. Virgin beetles were selected from a polymorphic laboratory population and mated to determine the inheritance of wing-cover color. The crosses and results were as follows:

a. Deduce the genetic basis of wing-cover color in this species.
b. Write the genotypes of all parents and progeny as completely as possible.
See answer

Solution
a. These data seem complex at first, but the inheritance pattern becomes clear if we consider the crosses one at a time. A general principle of solving such problems, as we have seen, is to begin by looking over all the crosses and by grouping the data to bring out the patterns. One clue that emerges from an overview of the data is that all the ratios are one-gene ratios; there is no evidence of two separate genes being involved at all. How can such variation be explained with a single gene? The obvious answer is that there is variation for the single gene itselfthat is, multiple allelism. Perhaps there are three alleles of one gene; let's call the gene w (for wing-cover color) and represent the alleles as wg, wb, and wt. Now we have an additional problem, which is to determine the dominance of these alleles.

Cross 1 tells us something about dominance because the progeny of a blue × green cross are all blue; hence, blue appears to be dominant to green. This conclusion is supported by cross 5 because the green determinant must have been present in the parental stock to appear in the progeny. Cross 3 informs us about the turquoise determinants, which must have been present, although unexpressed, in the parental stock because there are turquoise wing covers in the progeny. So green must be dominant to turquoise. Hence, we have formed a model in which the dominance is wb > wg > wt. Indeed, the inferred position of the wt allele at the bottom of the dominance series is supported by the results of cross 7, where turquoise shows up in the progeny of a blue × green cross.

b. Now it is just a matter of deducing the specific genotypes. Notice that the question states that the parents were taken from a polymorphic population; this means that they could be either homozygous or heterozygous. A parent with blue wing covers, for example, might be homozygous (wb/wb) or heterozygous (wb/wg or wb/wt). Here, a little trial and error and common sense are called for, but by this stage, the question has essentially been answered and all that remains is to "cross the t's and dot the i's." The following genotypes explain the results. A dash indicates that the genotype may be either homozygous or heterozygous in having a second allele further down the allelic series.
See question

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