How many Cousins does our DNA show?

+6 votes
I was just wondering Ancestry is showing me 5-7 cousins with my DNA.

How many cousins are we showing we are DNA here on Wikitree and how do I know if I am related to a member of Wikitree?


asked in The Tree House by Taylor Worthington-Gilchrist G2G6 Mach 6 (67.8k points)
retagged by Ellen Smith
The answers below are correct for our physical reality, how many cousins theoretically we have, but if you're asking how many cousins you will match on WikiTree, that's completely limited by how many profiles you have entered, and how many profiles that others have entered that are linked to yours.  Unfortunately, that's often far less people than in real life.

In addition, for DNA linked cousins, we're also limited to only those that have tested, and that's a LOT less than in real life.  Keep adding profiles, and keep encouraging relatives, especially distant ones, to get tested.

2 Answers

+8 votes
Best answer

Hi, Taylor. This is actually a more complex question than it may at first seem. The reason is that there are a whole lotta assumptions and computations and probabilities involved, and the answer you get will depend upon where the data for the answer comes from. For example, AncestryDNA performs a form of phasing on DNA results that compares your results against population genotype models. MyHeritage is doing the same thing. They publish their own estimations of the benefits of using this method and claim improved accuracy in identifying small shared segments, but they don't publish the genotype databases they use, and there have been no peer-reviewed scientific research into the claims they make. Caveat emptor.

One study whose data I tend to trust was published in PLOS One. I know this table is probably going to get distorted, but let's give it a try. 

Cousins          # You Have          # Detectable by DNA          % Detectable

1                     7.5                       7.5                                      100%

2                     38                        38                                       100%

3                     190                      170.4                                   89.7%

4                     940                      431.5                                   45.9%

5                     4,700                   700.3                                   14.9%

6                     23,000                 943                                      4.1%

In brief, the study arrived at the number of cousins you would have by assuming two sets of grandparents and 2.5 total children per couple. That worked out to pretty well accommodate current population rates, and slower growth rates in some countries that had higher population rates in past generations. So it's a modestly conservative figure.

The number of those cousins that are detectable as matches by DNA is also somewhat conservative. But again, I personally have seen no peer-reviewed research that indicates a higher level of accuracy in detection. Claims of higher levels of detectability are made, but it's up to each of us to decide how much we can trust those claims at face value. A really quick example is this blog post.

After you enter that you've taken an autosomal DNA test on your own profile, WikiTree will, in overnight processing, look for other people who have also said they've taken an atDNA test and who connect to you through WikiTree's "paper-trail" of profiles. That search extends out to a maximum of eight relationship degrees: eight separate birth events with each of which the DNA gets recombined and commingled. That equates to 6g-grandparents up the tree, and 3rd cousins horizontally across the tree. From that chart above, you can see that there's about a 90% chance that 3rd cousins will share a detectable amount of DNA. But that percentage drops precipitously at the 4th cousin level; drops to under 50%. Personally, I understand why WikiTree had to draw the line somewhere, and 3rd cousins seems correct to me.

People who have recorded they've taken an autosomal DNA test will show up on the "DNA Connections" column at the right-hand side of profiles if they are no more distant to you, on "paper," than 3rd cousins. If the assumptions and info above are correct, Wikitree would display at most 236(ish) cousins (1st through 3rd), not counting parents/grandparents, siblings, aunts/uncles, etc. About 20 of those 236 would not share enough DNA with you to be detectable. Of course, we haven't been able to talk all our 3rd cousins into being DNA tested, so the actual number of cousins "linked" is going to be much lower than 236.  :-)

answered by Edison Williams G2G6 Pilot (171k points)
selected by Rosemary Jones

Thank you very much

+4 votes
Not sure if I am understanding your question correctly Taylor, but on Ancestry I have 407 pages of matches, which at 50 to a page gives me over 20000 matches.
answered by Lynda Crackett G2G6 Pilot (617k points)

Boy you sure got a lot more matches than I did on Ancestry.

Thank you

Taylor, a lot of folks get confused and think the number of 4th cousins shown on the front page is their number of matches. If that is the number you were looking at hit the View All Matches button and you will see thousands more, but it does not show the total. There are 50 matches on each page.

Thank you for explaining that to me.

You are welcome. Have fun exploring your matches.

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