Chapter 19, Problem 22P

Suppose a polygenic system for producing color in ker-nels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, GI and 02, MI and M2, and T1 and 72. The phenotypic effects of the three genotypes of the G gene are GIG, = 6 units of color, G1G2 = 3 units of color, and G2G2 = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype GIGIMIMITITI a total of 18 units of color and a plant with the genotype G2G2M2M2T2T2 a total of 3 units of color.

a) How many units of color are found in trihybrid plants?

b) Two trihybrid plants are mated. What is the expected proportion of progeny plants displaying 9 units of color? Explain your answer.

c) Suppose that instead of an additive genetic system, kernel-color determination in this organism is a thresh-old system. The appearance of color in kernels requires nine or more units of color; otherwise, kernels have no color and appear white. In other words, plants whose phenotypes contain eight or fewer units of color are white. Based on the threshold model, what proportion of the F2 progeny produced by the trihybrid cross in part (b) will be white? Explain your answer.

d) Assuming the threshold model applies to this kernel-color system, what proportion of the progeny of the cross GIG2M1M2T2T2 x G 1 G2A/1/1/2Ti T2 do you expect to display colored kernels?