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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics
Solved Problem

3.

In a large natural population of Mimulus guttatus, one leaf was sampled from each of a large number of plants. The leaves were crushed and subjected to gel electrophoresis. The gel was then stained for a specific enzyme X. Six different banding patterns were observed, as shown in the following frequencies:

a. Assuming that these patterns are produced by a single locus, propose a genetic explanation for the six types.
b. How can you test your idea?
c. What are the allelic frequencies in this population?
d. Is the population in Hardy-Weinberg equilibrium?
See answer

Solution
a. Inspection of the gel reveals that there are only three band positions: we will call them slow, intermediate, and fast. Furthermore, any individual can show either one band or two. The simplest explanation is that there are three alleles of one locus (let's call them AS, AI, and AF) and that the individuals with two bands are heterozygotes. Hence, 1 = S/S, 2 = I/I, 3 = F/F, 4 = S/I, 5 = S/F, and6 = I/F.
b. The hypothesis can be tested by making controlled crosses. For example, from a self of type 5, we can predict 1/4 S/S, 1/2 S/F, and 1/4 F/F.
c. The frequencies can be calculated by a simple extension of the two-allele formulas. Hence
d. The Hardy-Weinberg genotypic frequencies are

which are precisely the observed frequencies. So it appears that the population is in equilibrium.

See question

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