Two different female Drosophila were isolated, each h...

Question

Two different female Drosophila were isolated, each heterozygous for the autosomally linked genes b (black body), d (dachs tarsus), and c (curved wings). These genes are in the order d–b–c, with b being closer to d than to c. Shown here is the genotypic arrangement for each female along with the various gametes formed by both:

Identify which categories are noncrossovers (NCOs), single crossovers (SCOs), and double crossovers (DCOs) in each case. Then, indicate the relative frequency in which each will be produced.

Genotypic arrangements and gamete types from two heterozygous females showing crossover categories and frequencies.

Identify which categories are noncrossovers (NCOs), single crossovers (SCOs), and double crossovers (DCOs) in each case. Then, indicate the relative frequency in which each will be produced.

Accepted Answer

Hi everyone, welcome back. Here's the next problem. It says the genes for short art is a. S. Black body B. And brown eyes, B. W. Are present on chromosome two of Drosophila. The order of these genes are as followed. S. B. B. W. Identify which the following gametes is not a result of a single crossover. In a female with the S. B. B. W plus slash S plus B plus B. W. Genotype. First of all, it's important to know there's a female in Drosophila crossing over occurs only in females. Now, let's think about what Her two chromosomes look like. So on this first chromosome we know the order that these genes go in. We have this S. A little A B. L. E. O. And A B. W. Plus value on the other chromosome of an S. Plus B. Plus and B. W. And we recall that there if there are no crossovers, she can produce only two types of gametes here. With. Since these are on the same chromosome, the first being S. B. B. W plus and the other one being S plus B plus B. W. So those are the two gametes are there are no crossovers. So any gammy that looks different from this has had at least one crossing over event. Take place. So with that in mind let's look at our answer choices. Choice A says S. B plus B. W. Plus. So let's look at our little chromosome diagram here and we see that S and B plus are occurring together. Then there must have been a cross over here between the S and B locations to allow them to be together, then we have B. W plus, well, B plus and B W plus are together on the same gamut, there must have been another crossover between the B and B W. G locations. So Choice A has two crossovers. So it looks like we've already hit on our correct answer, since we have to say which of the following is not a result of a single crossover. So it looks like choices correct. So I'm gonna go ahead and highlight that, but we'll just look at our other choices to make sure. So let's just erase this little diagram and look onto the next one. Choice B says S plus B plus B, W plus. Well as plus and B plus occur together on the same um chromosome here. But for them to occur with B W plus, there must have been a cross over here between the b m, b W, but that's just a single crossover. So that cannot be our answer. Now, we'll look at choice, see, oops, accidentally get rid of that, I'm there and Choice E says S, B, B W. So S and B occurred together. But for B and B w occur together, we must have had a cross over here between B and B W. So we've got a single crossover and choice C. So that's not our answer. Now, we're gonna look at choice D. D. Has S plus B and B W. Plus, so S. Plus and B. For them to occur together, there must have been a cross over here between S. And B. But then for B and B W plus to be together, they were together on the original chromosome, so no crossover needed there. So again, we have just a single crossover and Joyce de. So which of the following gametes is not a result of a single crossover choice A. S. B plus B. W. Plus, because it's a result of two crossovers. See you in the next video.

Key Concepts

Genetic Linkage

Types of Crossovers

Recombination Frequency

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