Biology Forums - Study Force

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.


Part A
How many units of color are found in trihybrid plants?
A) 9 units of color
B) 1 units of color
C) 3 units of color
D) 12 units of color

Part B
Two trihybrid plants are mated. What is the expected proportion of progeny plants displaying 9 units of color?
Express your answer as a fraction (example 3/4).

Expected proportion:

Incorrect Answers: 8/64 & 20/64


Part C
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 (b) will be white?

A) 1/8
B) 22/64
C) 1/4
D) 24/64

Part D
Assuming the threshold model applies to this kernel color system, what proportion of the progeny of the cross G1G2M1M2T2T2 × G1G2M1M2T1T2 do you expect to display colored kernels?

A) 14 of the progeny will be white and 34 will be colored.
B) 12 of the progeny will be white and 12 will be colored.
C) 34 of the progeny will be white and 14 will be colored.
D) None of the progeny will be colored.

Post Merge: 9 years ago

I was thinking that Part B could be 18/64 but I want to be definite before I answer again.

Post Merge: 9 years ago

Part C is 22/64

Content hidden

Thank You

Did you get Part B?

So far For Part B:   it's NOT 1/64

teacher's hint:
Keep in mind what the expected frequencies of heterozygous genotypes are at any one locus. (Is it really 1/4??).

No, Ive tried
8/64
20/64
and 18/64
and you said its not 1/64.

Post Merge: 9 years ago

i think it may be 1/8

From the textbook (page 42)
The probability that offspring of a trihybrid self-fertilization will be trihybrid is
(1/2) (1/2) (1/2) = 1/8

Yay, thats what I was thinking. Thank You !

Thank you

thank you