In a cross in Drosophila, a female heterozygous for the autosomally linked genes a, b, c, d, and e (abcde/ +++++) was testcrossed with a male homozygous for all recessive alleles. Even though the distance between each of the loci was at least 3 map units, only four phenotypes were recovered, yielding the following data:
Why are many expected crossover phenotypes missing? Can any of these loci be mapped from the data given here? If so, determine map distances.

Question

In a cross in Drosophila, a female heterozygous for the autosomally linked genes a, b, c, d, and e (abcde/ +++++) was testcrossed with a male homozygous for all recessive alleles. Even though the distance between each of the loci was at least 3 map units, only four phenotypes were recovered, yielding the following data:

Why are many expected crossover phenotypes missing? Can any of these loci be mapped from the data given here? If so, determine map distances.

Table showing four Drosophila phenotypes with corresponding fly counts totaling 1000: 440, 460, 48, and 52 flies.

Why are many expected crossover phenotypes missing? Can any of these loci be mapped from the data given here? If so, determine map distances.

Accepted Answer

Hi everyone. Welcome back. Let's look at our next problem here. It says blank is a mechanism in evolutionary genetics that occurs when irreversible harmful mutations accumulate in an a sexual population in the absence of recombination. So we imagine our population a sexually reproducing. So in general the offspring are clones of the parents. Aside from any mutations that arise um why absence of recombination is important is that recombination is in a sexual reproduction. Recombination during mitosis is sort of an error approving mechanism. But without that um each generation of a sexually reproducing population will inherit any mutations in the parental generation and then on top of that any mutations that arise in its own generation are added to that parental load. So you end up having this accumulation over the generations of more and more and more irreversible harmful mutations, assuming that reverse mutation is rare. So that mechanism referring to that process is choice D Mueller's ratchet is what refers to that notion. Now let's look at our other answer choices to determine why they are not the correct term. Choice A is genetic hitchhiking. It's kind of a funny term. And we can recall that that refers to the process where you have one gene subject subjected to a selective sweep. So the sense of natural selection acting on a certain gene to favor one over another, any neighboring linked genes will change allele frequency as a result. So even if the neighboring gene is not subject to that natural selection process. If it's linked to a gene that is it's a little frequency will change as a result of being linked to the gene that's being acted upon by natural selection. So the to the non selected gene is being is hitchhiking along. It's a low frequency being affected by the selection acting on the other one. But not the term we're looking for. So we're gonna go ahead and eliminate that. Then let's talk about choice be the fisher Mueller model and the fisher Mueller model says that sexual activity reduces competition between advantageous mutations developed in various backgrounds. Um And that's because it can bring beneficial mutations together. So because in sexual reproduction we have a lot of recombination instead of having um one part of the population which has one beneficial mutation, another part of the opposition has another beneficial mutation and having them compete against each other because of the possibilities of recombination and sexual reproduction. It can bring together those beneficial mutations in the offspring, reducing competition and giving them an extra bonus. Um But fisher Mueller model is not what we're looking at here, because that specifically refers to populations that reproduce sexually. So not what we're talking about here. So we can eliminate that. And then finally we've got the choice c right fisher model and the right fisher model just drink that down a little. So it fits there we go. There is a model in which you have a population where generations do not cross over and each copy of a gene identified in the current generation is chosen independently at random from all copies of the genes in the previous generation. In short, it means it's a population with no natural selection. So you would see that when you have things like genetic genetic hitchhiking, so Elise getting swept along by natural selection on other alleles. Um But again you have it's as opposed to a population we're seeing natural selection added. And right fisher model these generations, the alleles that are um brought along in the next generation are chosen by random chance. So so sometimes you see this connected with genetic drift where you see a population of small population where you've randomly chosen a group of alleles to be continued and there the frequency of their occurrence in the population has been affected by chance. But this is not referring to an accumulation of mutations as we're looking for in a problem here. So, right fisher model is not the mechanism that we're looking for. So again, choice D muller's ratchet is a mechanism in evolutionary genetics that occurs when irreversible, harmful mutations accumulate in an a sexual population in the absence of recombination. See you in the next video

Verified video answer for a similar problem:

Key Concepts

Genetic Linkage and Recombination

Testcross and Phenotypic Ratios

Interference and Double Crossovers