Suppose a polygenic system for producing color in kernels of a gr...

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 G₂, 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.

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 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.

Accepted Answer

Hey, everyone. Let's take a look at this question together. A plant's seed color is determined by two genes A and B each with two alleles. The A gene has three genotypes. Homozygous dominant A heterozygous A and Homozygous recessive A with phenotypic effects of 86 and four units of color respectively. And the B gene has similar effects. If C color is determined by a threshold system where at least 10 units of color are needed for purple seeds, what proportion of the F two progeny will be purple? Is it answer choice? A cannot be determined. Answer choice B one out of nine, answer choice C eight out of nine or answer choice D six out of nine. Let's work this problem out together to try to figure out what proportion of the F two progeny will be purple. So from the question, we note that the genotypes four A, the homozygous dominant A heterozygous A and homozygous recessive A have the phenotypic effects of 86 and four units of color respectively, which means that we have that homozygous dominant A heterozygous A and homozygous recessive A. And we also know that the B gene has similar effects, which means that we have that homozygous dominant, four B, the heterozygous four B and the Homozygous recessive four B. And we know that the homozygous dominant has eight units of color. The heterozygous has six units of color. And the homozygous recessive has four units of color. And here you can see on the screen that we have two punt squares, one for the A gene and one for the B gene that shows the different possible genotypes from a cross of two heterozygous individuals, which when the A gene and the B gene are combined, we have the following genotypic possibilities which you can see in the image on your screen. And since we know how many units of color are displayed for the homozygous dominant, the heterozygous and the homozygous recessive or the A and B genes, we are able to calculate how many units of color are displayed for each genotype, which can be calculated. And we have in the image on the screen, all of the totals of the units of color for each of the genotypes. And since we want to determine the proportion of the F two progeny that are purple, which purple is determined by having at least 10 units of color, we are able to determine that out of nine possible phenotypes. We have 1234567, eight out of nine that have at least 10 units of color, which are needed for purple seeds So that means that the proportion of the F two progeny that will be purple is eight out of nine. So answer choice C eight out of nine is the correct answer. I hope you found this video to be helpful. Thank you and goodbye.

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