Effect of cousins marrying cousins

[thetick]

So my answer to can Cousins marrying Cousins effect your DNA results?, would be; Depending on the level of Endogamy down the generations; a resounding Yes.

I agree as following groups have seen this:

Jewish, Mennonites, Pakistani, Acadian. Pretty much any group that has cousin marriages over many generations.

[LadyAlaise]

In my humble opinion...

There seems to be some dispute among the experts on this, but several have told me that a history of cousin marriage, especially when combined with very large family size, can "convince" the matching algorithm that the relationship is closer than it really is. This is certainly the case with descendants of my own French-Canadian ancestry.

I would agree; My Dad is Acadian French; I had him tested first via Ancestry; before I got his results I already knew by doing my paper trail Research (thank god for records and Stephen White) that my grandparents were cousins many ways; hitting each step along the way from 3rd through 11th cousins; even 8th cousins two different ways all through their lines; kind of an accepted fact in Acadian Genealogy. So I wondered how this would affect my Dad's and my own DNA; I had a good indication of how when I got my own matches on FTDNA; my TOP match (aside from my folks whom I have now transferred over) is a known 4th cousin once removed that FTDNA places at 2nd cousin-4th cousin range; we share like 4 or 5 sets of common ancestors; then I got my Dad tested at Ancestry; well I got his results and nearly chortled my coffee all over my laptop; he has 3,384 DNA Matches at 4th cousin or closer!
I like to call this the 'Fold-over effect'; like making a Japanese Samurai sword; folding the steel to make it stronger; having a lot of Endogamy can really confuse the DNA matching.
This all compared to my (mostly Irish; and Not Acadian thank god) Mom who only has 114 DNA matches on Ancestry.
LOL
So my answer to can Cousins marrying Cousins effect your DNA results?, would be; Depending on the level of Endogamy down the generations; a resounding Yes.

[Alexandrina]

PITCAITN ISLANDERS & CHRISTIAN FLETCHER-To Norfolk Island

What were their ancestral breakdowns like, just out of curiosity?

The Pitcairner Islanders were/are YDNA wise descended from a small group of Englishmen who were mutineers against Captain William Blight of HMS Bounty. Bligh became a Governor of New South Wales Colony-(Australia). There were also a handful of Pacific Islanders- Tahitian /Samoan males and the nine English/Scot/Irish mutineers.

The 10-11 women were Pacific Islanders. This unique group intermarried over the past 200+ years & are now represented by communities living on Pitcairn & Norfolk Islands. Many members have recently departed for Australia , New Zealand and elsewhere however. Various newcomers added to the gene pool especially George Hun Nobbs , who became the Minister to the community.

The Pitcairn/Norfolk Islanders are a unique group with many members descended from the infamous Christian Fletcher, leader of the mutineers on the Bounty. His family are very well documented around Cockermouth area Cumberland England.

Founding surnames of the males were:
Buffett; Christian; Quintal; McCoy; Adams & Young. (Mills -line extinct?) Evans- & Nobbs & others later.

It would be very interesting indeed to see autosomal DNA chromosome browser clips of the autosomal results within this very endogamous population. The NOBBS YDNA would be particularly interesting for those descended from that line.

Alexandrina.

Alexandrina.

[David Guetta]

I managed to get the one Pitcairn person take an autosomal test and compare that person to two people whose 1 grandparent was from Norfolk island, of which was resettled by Pitcairn people in 1850.

What were their ancestral breakdowns like, just out of curiosity?

[mamoahina]

I managed to get the one Pitcairn person take an autosomal test and compare that person to two people whose 1 grandparent was from Norfolk island, of which was resettled by Pitcairn people in 1850.

[mamoahina]

A few words to look up...

The phenomenon is called 'endogamy'.

The impact on DNA and relationship calculation is referred to as 'Founder Effect' or 'Pedigree Collapse'.

It still baffles me why people use founder population/effect and population bottleneck interchangeably.

A lot of populations start from founder's population which I've been told even by a population geneticist that a founder's is a population bottleneck which I understand. But my own population have gone through, apparently, several population bottlenecks and the most recent one was in the 1790s. It declined since but started to go back up after 1900.

I've also seen this with Pitcairn people and how in 1850 they went through a severe bottle necking even though they started with a founder's population in 1790.

[sarahmj]

I think this issue is affecting my results to a degree as well.
My mother's ancestry is from Malta which is a very endogamous population. Her paternal grandparents are 1st cousins and her maternal great grandparents are first cousins. Then after phasing my Mum's DNA with my Grandma it seems she shares about 32cm with the side my Mum inherited from her Father (can't get his DNA since he died a long time ago). So not sure how my grandparents are related but it seems they are cousins somewhere not too far back.
I do have a few matches where there are many segments but the size of the segments are smallish....so probably not a close match.

[mamoahina]

Yes, that's also possible, but statistically I would think it much more common for the segments to fragment than to merge. So for any segment that ends up more intact there will probably be multiple fragmented pieces. Since we are talking statistics though, for any particular case it can easily go against the norm.

Correct. It looks more fragmented, not necessarily merging and appears more like a comb with missing teeth.

[Adam Nisbett]

And recombination can make 2 small segments one larger segment again...[not necessarily x + y, but larger than either alone]

Yes, that's also possible, but statistically I would think it much more common for the segments to fragment than to merge. So for any segment that ends up more intact there will probably be multiple fragmented pieces. Since we are talking statistics though, for any particular case it can easily go against the norm.

[hansonrf]

In general based on what I understand of how DNA recombination works, cousin marriages end up giving you a larger dose of the common ancestry (since they would show up multiple times in your family tree this makes sense) but it's more likely to show up in multiple segments rather than one long segment. If there's a repeated history of cousin marriages you may end up with lots of shared DNA that's all broken into small segments.

And recombination can make 2 small segments one larger segment again...[not necessarily x + y, but larger than either alone]

[Adam Nisbett]

In general based on what I understand of how DNA recombination works, cousin marriages end up giving you a larger dose of the common ancestry (since they would show up multiple times in your family tree this makes sense) but it's more likely to show up in multiple segments rather than one long segment. If there's a repeated history of cousin marriages you may end up with lots of shared DNA that's all broken into small segments.

[K. L. Adams]

My wife's, father has a couple of double first cousins. Young folks in little southern towns did not stray far, especially if they only had a mule It will be interesting to see how family finder works for this particular line.

[crossover]

There seems to be some dispute among the experts on this, but several have told me that a history of cousin marriage, especially when combined with very large family size, can "convince" the matching algorithm that the relationship is closer than it really is. This is certainly the case with descendants of my own French-Canadian ancestry.

well in the case of my grandpa(whose parents are related in multiple ways and happen to be second cousins)gedmatch incorrectly predicted them to be within the 3rd-4th cousin range

[Dawn]

I have quite a few generations of 2nd cousins marrying.
I have a man who had shared cM's of approximately 70. When we dug into our tree, I found out he's my third cousin 2 times removed. I'm assuming we should be sharing way less DNA than we do.

[hansonrf]

A few words to look up...

The phenomenon is called 'endogamy'.

The impact on DNA and relationship calculation is referred to as 'Founder Effect' or 'Pedigree Collapse'.

It occurs in many 'tight-knit' communities, most notably where religious beliefs or customs or other constraints led to marriages between cousins to some degree.