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Grossvater
05-17-2019, 09:05 PM
So I compared the DNA of my wife and a friend of ours on the GEDMATCH autosomal calculator. To my surprise, they shared small portions of DNA on five different chromosomes. The sections are from 2.5 cM to 3.8 cM long. With such small pieces of shared DNA spread over so many different chromosomes, approximately how far back would they be related?

geebee
05-20-2019, 12:40 AM
So I compared the DNA of my wife and a friend of ours on the GEDMATCH autosomal calculator. To my surprise, they shared small portions of DNA on five different chromosomes. The sections are from 2.5 cM to 3.8 cM long. With such small pieces of shared DNA spread over so many different chromosomes, approximately how far back would they be related?

It can be very hard to tell with very small segments. One reason is that small segments have a low likelihood of a crossover occurring within the segment. "2.5 cM" and "3.8 cM" represent probabilities below 3% and 4%, respectively. So clearly segments this size could be passed on quite a number of generations without recombination. (And each time a segment escapes recombination, the probability the next time is the same as it was the generation before.)

As to there being multiple shared segments of this size, that might mean nothing more than that there are several distant common ancestors -- but each segment can easily reflect a different ancestor.

Finally, with small segments there's also a high likelihood that they aren't actually "identical by descent", but "identical by chance".

EDIT:

Peter Ralph and Graham Coop wrote a paper entitled "The Geography of Recent Genetic Ancestry across Europe". https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001555

They examined SNPs in the "Population Reference Sample (POPRES) dataset" in order to identify small shared DNA segments. What they found was that people from virtually anywhere in Europe shared DNA with people from all over Europe.


Pairs of individuals in this dataset were found to share a total of 1.9 million segments of IBD, an average of 0.74 per pair of individuals, or 831 per individual. The mean length of these blocks was 2.5 cM, the median was 2.1 cM, and the 25th and 75th quantiles are 1.5 cM and 2.9 cM, respectively. The majority of pairs sharing some IBD shared only a single block of IBD (94%). The total length of IBD blocks an individual shares with all others ranged between 30% and 250% (average 128%) of the length of the genome (greater than 100% is possible as individuals may share IBD blocks with more than one other at the same genomic location).

If your wife's ancestors were in the same area for an extended period of time, it wouldn't surprise me if she, too, shares DNA with people from all over that region -- including people with whom she shares no known ancestry.

One of my communities at Ancestry is "Eastern Kentucky & Northeast Tennessee Settlers". That community has over 367 thousand members -- and while I might not share DNA with all of them, I definitely share DNA with a lot of them.

In fact, when I did a search of my DNA matches with "Kentucky" as "birth location", I came up with over 80 in the "4th cousin or closer" category. Keep in mind that this list only includes folks who identified Kentucky as an ancestral birth location.

Grossvater
05-22-2019, 07:58 PM
It can be very hard to tell with very small segments. One reason is that small segments have a low likelihood of a crossover occurring within the segment. "2.5 cM" and "3.8 cM" represent probabilities below 3% and 4%, respectively. So clearly segments this size could be passed on quite a number of generations without recombination. (And each time a segment escapes recombination, the probability the next time is the same as it was the generation before.)

As to there being multiple shared segments of this size, that might mean nothing more than that there are several distant common ancestors -- but each segment can easily reflect a different ancestor.

Finally, with small segments there's also a high likelihood that they aren't actually "identical by descent", but "identical by chance".

EDIT:

Peter Ralph and Graham Coop wrote a paper entitled "The Geography of Recent Genetic Ancestry across Europe". https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001555

They examined SNPs in the "Population Reference Sample (POPRES) dataset" in order to identify small shared DNA segments. What they found was that people from virtually anywhere in Europe shared DNA with people from all over Europe.


Pairs of individuals in this dataset were found to share a total of 1.9 million segments of IBD, an average of 0.74 per pair of individuals, or 831 per individual. The mean length of these blocks was 2.5 cM, the median was 2.1 cM, and the 25th and 75th quantiles are 1.5 cM and 2.9 cM, respectively. The majority of pairs sharing some IBD shared only a single block of IBD (94%). The total length of IBD blocks an individual shares with all others ranged between 30% and 250% (average 128%) of the length of the genome (greater than 100% is possible as individuals may share IBD blocks with more than one other at the same genomic location).

If your wife's ancestors were in the same area for an extended period of time, it wouldn't surprise me if she, too, shares DNA with people from all over that region -- including people with whom she shares no known ancestry.

One of my communities at Ancestry is "Eastern Kentucky & Northeast Tennessee Settlers". That community has over 367 thousand members -- and while I might not share DNA with all of them, I definitely share DNA with a lot of them.

In fact, when I did a search of my DNA matches with "Kentucky" as "birth location", I came up with over 80 in the "4th cousin or closer" category. Keep in mind that this list only includes folks who identified Kentucky as an ancestral birth location.

To further complicate the discussion, our friend is 87% Native American with most of his ancestry among the Shoshone. His father's people were from Acoma Pueblo in New Mexico. My wife's nearly 30% Native American ancestry is primarily from Nuevo Leon & Tamaulipas in Northern Mexico although several of her ancestral lines moved there from farther south. 23andMe tells us his autosomal DNA is more southly shifted than our relatives with Northern Plains Native American ancestry. Four of the small pieces that he and my wife share are almost certainly Native American and one is Iberian. His small amount of European ancestry is almost all Iberian and he carries a European Y chromosome.

This is pure speculation but could all of these pieces of shared DNA come from early New Mexico settlers from Nuevo Leon who were already "mestizo" when they arrived? The Martin-Serrano family (the progenitors of the vast Martinez clan) comes to mind who had a founding ancestor in New Mexico who was a mestizo gal from Jalisco. Or could it be that both of them descend from very ancient people who were part of the founding population of Mexico and parts south? Our Shoshone friend was just as astonished as I was to find out he has DNA matches with people in Guatemala and even possibly in Peru! He has absolutely no knowledge how any folks from that part of the world ended up in his family tree. I've only been able to get most of his lines back to the mid-1800s. I feel very fortunate to get one line back into the late 1700s only because he descends from a very famous Shoshone Chief.

The DNA is speaking but I can't seem to figure out what it is saying. Any ideas?

Chad Rohlfsen
05-22-2019, 11:19 PM
One thing to consider is that Shoshone are Uto-Aztecans, and the Tamaulipas natives probably have this ancestry too, along with being Mayan speakers. That could be the shared ancestry with your wife and Guatamala. As far as the Pueblo, that's hard to say. I believe an ancient New Mexican genome had connections with Peruvians, but seemed odd with no linguistics connection.

Hope that helps.