Three independently assorting STR markers (A, B, and C) are used ...

Question

Three independently assorting STR markers (A, B, and C) are used to assess the paternity of a colt recently born to a quarter horse mare. Blood samples are drawn from the mare, her colt, and three possible male sires (S₁, S₂, and S₃). DNA at each marker locus is amplified by PCR, and a DNA electrophoresis gel is run for each marker. Amplified DNA bands are visualized in each gel by ethidium bromide staining. Gel results are shown below for each marker.

Evaluate the data and determine if any of the potential sires can be excluded. Explain the basis of exclusion, if any, in each case.

Accepted Answer

Hi, everybody. Let's look at our next question. It says, suppose we have the graph results of the electrophrenic analysis of the markers. We want to investigate. How can we identify if a gene has a heterozygous genotype choice A, they have one peak choice B, they have two peaks, choice C, they have three peaks or choice D they have no peak. Well, when you're looking at an electrophrenic analysis of a particular marker, each peak corresponds to a specific number of repeats in the marker area you're looking at. And so if you have two peaks, that means that the two chromosomes have a different numbers of repeats or in other words, they have different alleys of the same marker gene. So when you have two different, all the genotype would be heterozygous. So that equals heterozygous genotype. So to identify if a gene has a heterozygous genotype, when we look at the graph results of that electrophrenic analysis, we will get to choice B, they have two peaks as our answer. If we see one peak, as we see in choice A in a specific marker region, that would mean that it is a homozygous genotype because both chromosomes have the same number of repeats. That's what leads to that single peak since each peak corresponds to a specific number of repeats. But that's why choice a isn't correct. Do I see three peaks? We would not see because we don't have since there aren't three chromosomes of each type. Um However, sometimes you might see a very tiny third peak if you have a heterozygous genotype, but there's some sort of PC R error that might cause a tiny little third peak. But again, that's not going to state a heterozygous genotype. So that's not our answer. And then of course choice d they have no peak is not going to be giving you a heterozygous genotype. So again, looking at graph results of an electrophrenic analysis, how can we identify if a gene has a heterozygous genotype choice B? They have two peaks. See you in the next video.

Key Concepts

Short Tandem Repeats (STRs)

Polymerase Chain Reaction (PCR)

DNA Electrophoresis

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