A couple planning their family are aware that through the past th...

Question

A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.

Would you advise the woman that she will have to bring each pregnancy to term to determine whether the fetus is normal? If not, what else can you suggest?

Accepted Answer

Hi, everyone. Welcome back. Let's look at our next question. It says regarding inversion, which of the following statements is incorrect. And when we look at our answer choices, we see, we've got um questions asking our choices about um inversion loops crossing over um abnormal gametes. All those questions are sort of really complicated to think through um without I think a visual aid. So I'm just gonna put up real quick two diagrams to help us kind of refer back to as we evaluate these answers, remembering we're gonna have three true statements, but we're looking for the incorrect statement. So first, just the basic diagram to remind us what an inversion looks like. So here we have on the bottom, the original chromosome, here's the dot representing its centromere and the order of its genes originally. And then of course, here within these brackets, I have the inversion uh not including the centromere and have just reversed the order of this portion. Um We have two, you know, we're talking about meiosis here. We've duplicated our chromatis. So we've got um two homologous chromosomes here sort of just drawn, straightened out like that. And the second diagram I want to put up is the inversion loop. So during meiosis, as these two homologous chromosomes try and pair up, they're going to run into issues pairing up um where these genes are not in the same order. So my second kind of reference diagram here is the inversion loop that occurs when the two chromosomes try to pair up. So A and B no problem. But then for CD E to match up one of these chromosomes, in this case, I've just drawn the um original one has to loop around itself for these two gene, these three genes to line up. And then here we've got F at the end. All right. Well, you got that issues start to arise when we see maybe crossing over occurring within that inversion loop. So let's envision a crossover occurring right here between D and C and envision what kind of gametes are going to occur when you have a situation like that. So, imagine up here, we've got, you know, our, it's got an inversion but fairly normal um chromatid in that game. But when we come along here and look at the game meat that results from uh the next chromatid here, we see something different. We got A B our centromere coming up here, there's E but then we hit the crossover and you've crossed over down here onto this chromatid because these two chromatid have swapped at that crossing over point instead of continuing on to F, we now duplicate gene C gene D gene E B A second centromere and A. So we've now got a chromatid with two centromere and duplicated chromosome or duplicated genes. Excuse me. Well, when they, when you're going through meiosis, those spindle fibers are gonna attach to both centromere and kind of pull it apart. So you're gonna have little fragmentary portions of that chromatid, which will result in abnormal gamete and probably cause severe problems and may not lead to a viable offspring. So there's kind of our referral diagrams to help us look through these choices and evaluate them as to whether they're um correct or incorrect and look for our incorrect segment. That's our answer. So we start with choice A which says individuals carrying large inversions are more likely to produce abnormal gas. Well, this is definitely a true statement. Obviously, the bigger the inversion, um the more likely you are to cause significant issues, you'll have more likely to have crossover occurring um in those inversion loops because there's more space there more likely to include your centromere in this which causes definite issues. So the larger the inversion, the more likely you are to end up with abnormal gas. So it's a true statement. It's not our answer. Our next choice is b the phenomenon of crossing over within the inverted region during meiosis. One is the underlying cause of gamine abnormality and that is correct. As we could see, it's the crossing over that really causes major issues with duplication of parts of the chromosome deletion of others. This uh chromatid here that I highlighted doesn't even have an F gene and the duplication of the centromere is causing it to be pulled apart. If there was no crossover, you wouldn't have uh these abnormal gams occurring. So choice B also true and therefore not our answer. Then we're on to choice C crossing over in inversion loops results in deletion and duplication. And again, our little diagram there, we saw how both of those things result from a crossover within the loop. So that's also a true statement and not our answer. So now we are down to choice D which says if a crossover occurs, switch here within the inversion loop, a crossover within the inversion loop, the probability of producing abnormal chromosomes is very rare. Well, again, as we walk through this, looking back at this reference diagram, we can see that very rare is definitely not the case um crossing over and within the inversion loop will definitely lead to big issues. So when this was the one, we will choose for our incorrect statement regarding inversion, which of the following statements is incorrect. Choice D if a crossover occurs within the inversion loop, the probability of producing abnormal chromosomes is very rare. See you in the next video.

Key Concepts

Chromosomal Inversions

Karyotype Analysis

Genetic Counseling

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