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Understanding alleles

In this video, Dr Gavin Turbett explores the concept of alleles in further detail.
There is a range of possible alleles that can occur at each locus. So, for example, I’ve zoomed in here on just one locus, the locus here is D3S1358, and we know that this locus has 12 common alleles, and those alleles range from the designation allele 9 up to allele 20 and I’ve shown them there, and this particular individual happens to be have alleles 17 and 18. But if we tested a different person, they might have two completely different alleles. There are other possibilities. There are sometimes alleles that could be smaller than allele nine. They might even be occasionally people with alleles larger than allele 20. There are even variants that can occur in between.
But all of those are quite rare. So, those 12 common alleles between 9 and 20, most people will have some combination of two of those alleles, and at any one, locus, different people could actually have the same alleles simply by chance. So, here I’ve shown part of the DNA profiles of three people. These profiles have just been made up. So, I’ve got person1, person 2 and person 3, and I’ve just shown what is equivalent to the blue dye channel for those three people. So, in actual fact, of course, the DNA profiles would continue on to the right but we’re just showing a part of the profile to make it easier to see.
And this is how your DNA profiles would actually look once the data’s been interpreted and uploaded into a DNA database. So, we can see person 1 and person 2 at the Amelogenin locus have X and Y chromosomes, that tells us, of course, that they a male. Person 3 has two X chromosomes. Of course, we therefore know that person is female. So, we see that person 2 happens to be homozygous at the Penta E locus. They are a 15, 15 homozygote. Again, as I said before, this is perfectly normal and the people are heterozygous at all of the other loci. Again, this is perfectly normal. People will share alleles by chance, as we saw with D3.
There’s only about a dozen alleles that are typically seen, so it is inevitable that people must occasionally share alleles. In this example, we can see at D1S1656 locus, persons 2 and 3 both happen to have an allele 15. Likewise, at D6, persons 1 and 2 both happen to have an allele 20. Again, this is perfectly normal and will simply happen by chance. In some instances, the different people will share the entire locus, so you can see person 1 and person 2 at D13 are both 9,11 heterozygotes. Again, this is perfectly normal and it is inevitable that this will be seen by chance alone. And this is another very important point I’d like to make here.
The allele designations, the numbers that we are providing for each locus only apply to that locus. You can see here, for example, that person 1 has an allele 17 at D3 person 2 has an allele 17 but it’s at D1 locus. They are completely unrelated to each other and must never be compared. So, the allele 17 at D3 has absolutely nothing to do with the allele 17 at D1. Likewise, the allele 15 at D3 that person 2 has, has nothing to do with the with person 3’s allele 15 at D1 or the allele 15 at D6. They must never be compared to each other. It would be a very significant error to compare levels between different loci.

How many alleles can occur at a particular locus? Can two people have the same alleles at a specific locus? If yes, what does that signify? Does this affect the interpretation of DNA profiling results? Let us examine these questions with Dr Turbett in this video.

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