Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G1 and G1, M1 and M2 , and T1 and T2 . The phenotypic effects of the three genotypes of the G gene are G1G1 = 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 G1G1M1M1T1T1 a total of 18 units of color and a plant with the genotype G2G2M2M2T2T2 a total of 3 units of color. 1. Two trihybrid plants are mated. What is the expected proportion of progeny plants displaying 9 units of color?
2. Suppose that instead of an additive genetic system, kernel-color determination in this organism is a threshold system. The appearance of color in kernels requires 9 or more units of color; otherwise, kernels have no color and appear white. In other words, plants whose phenotypes contain 8 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 (1) will be white?
a. 1/8
b. 24/64
c. 1/4
d. 22/64