Mutations

[sarumhawk]

Hi all,

I've tried this question in other places with no joy, so here's hoping someone here will have an answer.

I think I'm right in saying that a marker can mutate in either direction so that someone with a repeat count of, say, 13 at DYS393 could have one descendant who has 14 at that marker and another with 12, but is it possible for a mutation to be reversed? Could a person with a repeat count of 13 have a descendant with 12 who then goes on to have a descendant of his own with the same marker mutated back to 13? Following on from this, would it be possible for the DNA of two unrelated family lines with the same surname to mutate towards each other with the genetic distance decreasing, rather than increasing?

Peter Carroll

[robe3b]

Hi all,

I've tried this question in other places with no joy, so here's hoping someone here will have an answer.

I think I'm right in saying that a marker can mutate in either direction so that someone with a repeat count of, say, 13 at DYS393 could have one descendant who has 14 at that marker and another with 12, but is it possible for a mutation to be reversed? Could a person with a repeat count of 13 have a descendant with 12 who then goes on to have a descendant of his own with the same marker mutated back to 13? Following on from this, would it be possible for the DNA of two unrelated family lines with the same surname to mutate towards each other with the genetic distance decreasing, rather than increasing?

Peter Carroll

Hi,
I think this paper might help to answer your question:

http://www.cstl.nist.gov/div831/strbase/pub_pres/Decker_YfilerMutationRate.pdf

[cacio]

Yes, you are right. I am not an expert on this, and someone else will be able to give more precise statements here, but markers can mutate in all directions, back and forth, so you can have occasional cases where STRs are overall similar, even though the two people actually belong to different haplogroups. However, the more markers you have, the less likely this is - that is, you can have random convergence on a few markers, but, probabilistically, it is extremely unlikely to have convergence on many markers.

cacio

[sarumhawk]

Thanks for your replies. I think the answer is that, whilst the scenarios I asked about are possible, it's all a game of probabilities.

Peter Carroll.

[tomcat]

There is a tendency of longer STR's to drop a repeat and shorter STR's to add a repeat, but it only a tendency. And it is possible for two originally distinct genotypes to come to resemble one another by mutational happenstance, to converge to an identity-by-state, as opposed to a more meaningful phylogentic/genealogical identity-by-descent. But even in such an instance the false similarity would most likely be less than perfect.

[ironroad41]

mutations

Yes it is a game of probabilities - but there are constraints. Goldstein, et.al. in a 2001 paper in Science, 291 showed that TMRCA can be predicted by any one dys loci. Now, he mentions that, in practice, many are used and averaged, but the result remains. This implies to me that if you have a set of entries with a common ancestor, the mutations that occur across the set are required to meet the constraint. To really see this though means you must have a very large data set, that is also very old so that the slow mutators like 426,388 etc. have had enough mutations. I also believe that the effect of someone having a mutation at a slow dys loci will affect the apparent mutation rate of the entire set of entries???