Does auDNA SNP's mutate?

+8 votes
104 views
I am not a biologist or a geneticist, nor do I pretend to even understand, what I should.  I think I  have a working knowledge of DNA, enough to be dangerous..

Having provided my lack of bonafides, might I posit my question.

What happens to auDNA after say the 10th generation

As I understand it, there is not enough bits of DNA left after the 9th generation (counting self as gen 1) to predict any sort of a relationship.

Eventually, with each succeeding generation, what bits there are, are halved, eventually the contribution is so negligible as to not be "readable".

I do understand that DNA is not precisely halved with each reproduction.

Anymore than my siblings and I receive the exact same chromosones from each parent, otherwise all siblings would be copies, (exception the Baldwin brothers)

But with a halving at each generation, it would seem that at some point in time a particular chromosone, would virtually disappear from a lineage.

To put it this way.  I descend from Charlemagne (supposedly, as perhaps do most western Europeans), however Charlemagne's auDNA will not show up in my profile, not even with triangulation.

While it might have disappeared completely in my line, another person could have a snippet in their genetic profile. But there is no way of Identifying which SNP('s) came from Charlemagne.

While it appears that some auDNA SNP's do in fact fall out during the reproductive process, over generations, new DNA is not created during the reproduction process, however there should be mutations, just as there are mutations in Y and mtDNA. (I'm merely theorizing, not postulating).

These mutations could (should?) appear as new SNP's, then again do SNP's mutate?

I apologize if I sound confused, because I am. I am really trying to grasp this science.

Why would two people who share a 4th great grandparent, and the same YDNA, not show up as 5th cousins?

And how can three people whose, common (paper) ancestor be a 7th or 8th great grandparent, show up on family finder as 4th cousins remote?

Yet at Y67, he shows up at a GD of 2 and 3 to these same people (they are nephew and uncle)

At Y67 he shows up at a GD of 0 to two people, whose common paper ancestor would be a 7th or 8th great grandfather.

But he does not show up as a Family Finder match on either of his Y67 GD=0.  On the other hand he shows up as a 4th cousin remote to two people whose common ancestor (if one's claimed genealogy is correct) would be a 7th - 9th great grandfather., but their YDNA match at Y67 is at a GD of 2 and 3.

I apologize for the confusion, but the confusion is all mine.
in The Tree House by Anonymous Farrar G2G6 Mach 1 (14.8k points)
edited by Maggie N.

5 Answers

+6 votes
Well, a SNP is a Single Nucleotide Polymorphism.  and typically the DNA tests produce values for something like 700,000 SNPs  and a given stretch of DNA must have 700+ identical SNPs to count as a match. And the 700,000 SNPs tested are only a tiny fraction of the total number of nucleotides in the human genome.  Now in each generation a small number of nucleotides are mutated, but not enough to change matches, especially larger ones.  most changes in matches are caused by crossovers of sections of DNA without any mutation.  And since only a tiny fraction of the total mutations in a genome are tested for, you can forget about mutations over the small numbers of generations  which we can matches for in an auDNAtest.
by Dave Dardinger G2G6 Pilot (406k points)
+4 votes
Two people who share a 4th great grandparent: Each child inherits about half their auDNA from mom and half from dad, but not always the same half. So after 6 generations, some of the 4th great grandkids might not have inherited any auDNA from that particular 4th great grandparent.

Three people with a common 7th or 8th great grandparent: In an endogamous community (small enough so that many cousins marry) there may be enough overlapping auDNA that cousins appear to be closer than they really are.
by Kay Wilson G2G6 Pilot (175k points)
+3 votes

Points to that other lively thread I started. I'd like to quote the OP: "Eventually, with each succeeding generation, what bits there are, are halved, eventually the contribution is so negligible as to not be "readable"."

So where is the DNA from that is each generation replacing 50% of the DNA I might ask. It can't appear from thin air. Nor is it constructed. It's from your other parent's ancestors, right?

As I said in the other thread, if we ignore the fact that we have Ahnenschwund (pedigree collapse) I fully agree that out of that endless (from a math standpoint) number of ancestors we lose the ancestral segments over time (without endogamy surely much earlier).

But

we all know for a given fact that pedigree collapse existed and happened many times in the history of us humans. The interested reader might want to check out this article on the world population over time: https://en.wikipedia.org/wiki/World_population

So my answer is no, your ancestral segment from beyond 10 generations is still existing with you, it came down not by one single path (or 1 out of your 512 ancestors at gen 10 - I hope I got the math right, not really checking it right now) because there weren't 512 different ancestors in the first place. Maybe 200, maybe 250 to 300. Who knows (I'm not an expert on pedigree collapse or world population models over time).

Hope this clarifies some. I really hope you guys read the Wikipedia article to understand how few people lived in former times (we're now over 7.4 billion on this planet, estimates are 11.2 billion by 2100), some highlights here:

  • The population of Europe was more than 70 million in 1340
  • The Black Death pandemic of the 14th century may have reduced the world's population from an estimated 450 million in 1340 to between 350 and 375 million in 1400
  • England's population reached an estimated 5.6 million in 1650, up from an estimated 2.6 million in 1500
  • Around 300 BCE, the population of India was between 100 million and 140 million
  • The population of India in 1600 was around 100 million. Hence, from 300 BCE to 1600 CD India's population was more or less stable.
For the old world (Europe) where most of us originate from:
It took more than 1000 year to go from 28 million to 50 million (CE 1 to 1000). It took another 500 years to get to 84 million. It takes another 250 years to double that to 163 million and we arrive in 1750, we most of us have many known ancestors already!
by Andreas West G2G6 Mach 5 (53.9k points)
edited by Andreas West
+2 votes
Maybe the "porcupine" graphs in this blog post will help you visualize the process. The white spaces show ancestral lines from which you inherited no DNA, but everything adds up to one complete genome. Whenever one ancestor drops out, that ancestor's spouse makes up the deficit.

https://gcbias.org/2013/11/11/how-does-your-number-of-genetic-ancestors-grow-back-over-time/
by Ann Turner G2G6 (8.5k points)
0 votes
Hi, William.  I'm not a geneticist, but I do have a research background in biochemistry and molecular biology.  In answer to your title question, yes, autosomal DNA, including SNP's does mutate over time.  Given that SNPs are just genomic locations (or loci, as geneticists call them) that tend to have slightly different sequence between populations, mutation is how SNPs came to exist in the first place.  That said, inheritable mutation is infrequent enough that it is probably negligible over any time scale that we are reasonably looking at.

I don't fully understand the rest of your questions, but that is probably just a matter of terminology.  I would suggest that, instead of thinking of DNA being "halved" or chromosomes disappearing, you think instead of DNA being hybridized and any given SNP being potentially (but not necessarily) diluted out over time.  Kind of like two decks of cards being shuffled together and separated into two again.  After multiple shuffles, you may or may not have a 10 of clubs, but the size of each deck remains the same.
by B Showalter G2G Crew (440 points)

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