Suppose a polygenic system for producing color in kernels of a grain is controll

Question

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

Explanation / Answer

Part 1=1/8 .the probablity that a offspring of atrihybrid self fertilization is (1/2) (1/2) (1/2)= 1/8

Part 2=22/64

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