Three strains of green-seeded lentil plants appear to ha...

Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

What proportion of the F₂ will have yellow seeds? Show your work.

Table showing F1 and F2 phenotypes of green-seeded lentil strains G1, G2, G3 crossed with yellow strain Y.
What proportion of the F₂ will have yellow seeds? Show your work.

Accepted Answer

Hi, everyone. Let's take a look at this practice problem together. A dominant allyl of a gene creates a product that results in blue pigment. And a dominant allyl of another gene acts on this product to produce green pigment. The presence of a recessive A lil in homozygous condition for the gene producing blue pigment masks the phenotypic expression of the second gene producing green pigment. What is the possible molecular explanation for this genetic system? The answer options are a one gene is blocking the expression of another B, these two genes are working in succession on a single path C. These two genes have the same basic effect and d either gene alone is producing a partial phenotypic effect. So a lot of words at this point, if you would like, go ahead and pause this video and re-read the question. I do have a diagram that I will put up on the screen that explains the first sentence of this question. So that diagram is up on the screen. Let me go ahead and describe it for you. So in this diagram, we have a precursor and when gene A is in the homozygous dominant or heterozygous genotype, it will take that precursor and produce a blue pigment. The second gene which is gene B, if it is homozygous dominant or heterozygous, it will take that blue pigment and produce green pigment. So this diagram describes that first sentence. Now, I am putting up a second diagram that describes the second sentence of the question. In this diagram on the screen, the again precursor in the presence of gene A when it is homozygous recessive will still create that blue pigment. However, when it is homozygous recessive, it masks the phenotypic expression of that green pigment. What that means is when gene B, no matter what the genotype is. Homozygous dominant heterozygous or homozygous recessive gene A in homozygous recessive genotype masks the production of the green pigment. So the second diagram describes what's happening in that second sentence. So what type of genetic system is this? This is describing an example of epitasis. Now recall what epitasis is. It's when two or more genes contribute to phenotype and their effects are not just additive. So let's take a look at the answer options. Again. A one gene is blocking the expression of another. We can eliminate option A gene A in homozygous recessive genotype masks, the phenotype of gene B. It does not block the expression C the two genes have the same basic effect is also an incorrect answer. Gene A produces blue pigment and gene B acts on the product of gene A we can also eliminate option D that either gene alone is producing a partial phenotypic effect. This is incorrect. They work together and contribute to the phenotype, which is the definition of epitasis genes. So, option D is also incorrect. The correct answer is option B, these two genes are working in succession on a single path. All right, everyone. I hope you found this helpful and I'll see you soon for the next practice problem.

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