On the Drosophila X chromosome, the dominant allele y⁺ p...

Question

On the Drosophila X chromosome, the dominant allele y⁺ produces gray body color and the recessive allele y produces yellow body. This gene is linked to one controlling full eye shape by a dominant allele lz⁺ and lozenge eye shape with a recessive allele lz. These genes recombine with a frequency of approximately 28%. The Lz gene is linked to gene F controlling bristle form, where the dominant phenotype is long bristles and the recessive one is forked bristles. The Lz and F genes recombine with a frequency of approximately 32%.

Using any genotypes you choose, design two separate crosses, one to test recombination between genes Y and Lz and the second between genes Lz and F. Assume 1000 progeny are produced by each cross, and give the number of progeny in each outcome category. (In setting up your crosses, remember that Drosophila males do not undergo recombination.)

Accepted Answer

Hi, everyone. Let's take a look at this practice problem together in the fruit fly, Drosophila. The genotype W plus for the red I lil is dominant to its a little w the white eye lil if a female Drosophila of a known genotype was crossed with a white eyed male fly genotype X W Y. 50% of the male and the female offspring were red eyed and 50% of the male and the female offspring were white eyed. Which of the following is the genotype of the unknown female fly. The answer options are a homozygous dominant B homozygous recessive C heterozygous and D none of these. So I have a picture to help us solve this problem. And I am putting that up on the screen right now. This picture is basically a pedigree of the cross between the unknown female and the white eyed male just drawn with fly pictures instead of the traditional squares and circles. The first row is the parental generation with the two flies, the unknown female and the white eyed male and the, the bottom row is the offspring. Now in a cross between our female and male, there are four possible offspring that are produced, half are female and half are male. So working from left to right, the first two flies are labeled female and the next two flies are labeled male. Now, together, we'll work through the problem and label this picture to answer the question. In the question, we are told that 50% of the male and female offspring had red eyes and 50% of the male and female offspring were white eyed. This means that one of the males and one of the females have red eyes. So working from left to right in the offspring row, let's give the first female offspring, red eyes and the first male offspring, red eyes. Now, we are also told that the parental, white eyed male genotype is X W Y. So let's write that above that male, white eyed fly since white eye color is recessive. And we know that from the question text, in order for any female Drosophila to display white eye color, their genotype must be homozygous recessive. So our second female offspring has white eyes and would have a homozygous recessive genotype X W X W. So let's write that above her. That means one of the unknown female parents, all lies must be recessive. We know this to be true because in order for the Drosophila female offspring to have those white eyes, they must inherit a recessive a lil from the female parent and the male parent. So let's write the recessive a lil above the unknown female parent. So we know one of her a little and we just need to figure out the other a little so far of the genotypes we've discussed, we've only talked about recessive white eye color, but we know that some of the offspring display red eyes. So that means that the unknown female parent must also have a dominant a lil for red eye color in order for the offspring to have red eyes. So let's write that dominant A lil above the female parent fly. So X W plus that means the unknown female parent's genotype is heterozygous. She must be able to pass on the dominant A lil for red eyes or the recessive white eye a lil to her offspring. And that means she will also have red eyes. So we'll fill in her eye color, red as well. Now, at this point, we could do a punt square if necessary to solve for the rest of the offsprings genotypes. But for this question, we have solved our an for our answer. So the correct answer is that the genotype of the unknown female fly is option C heterozygous. All right, everyone. I hope you found this helpful and I'll see you soon for the next practice problem.

Key Concepts

Genetic Linkage

Recombination Frequency

Drosophila Genetics

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