# Question about DNA testing **URL:** \ **Category:** Factual Questions\ **Created:** [July 14, 2021, 6:47pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490 "2021-07-14T18:47:42Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![jaycat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jaycat/32/3015_2.png) [@jaycat](https://boards.straightdope.com/u/jaycat)\ **Post date:** [July 14, 2021, 6:47pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/1 "2021-07-14T18:47:42Z")
A friend of mine was contacted by someone claiming to be their long-lost cousin. This person states that they got a 23&me DNA test, which ‘proves without a doubt’ that my friend’s uncle was this person’s father. The 23&me website says they can ‘build a family tree,’ and I’m trying to figure out how this is possible. How can they match this individual to my friend’s uncle (long deceased) without having a sample of his DNA? ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [July 14, 2021, 7:18pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/2 "2021-07-14T19:18:23Z")
All they would need is a sample from a known relative of the deceased uncle in order to make the link. ---
**Author:** ![zimaane](https://avatars.discourse-cdn.com/v4/letter/z/a3d4f5/32.png) [@zimaane](https://boards.straightdope.com/u/zimaane)\ **Post date:** [July 14, 2021, 7:20pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/3 "2021-07-14T19:20:11Z")
This sort of thing is pretty common. Your friend took a 23andMe DNA test. This other individual also took a 23andMe DNA test. 23andMe compared the two results, and found that the level of similarity in the DNA results indicated that the your friend and this other person had common grandparents. ---
**Author:** ![jaycat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jaycat/32/3015_2.png) [@jaycat](https://boards.straightdope.com/u/jaycat)\ **Post date:** [July 14, 2021, 8:27pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/4 "2021-07-14T20:27:20Z")
> [@zimaane](#): > > Your friend took a 23andMe DNA test. This other individual also took a 23andMe DNA test. My friend has never taken a DNA test. ---
**Author:** ![echoreply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/echoreply/32/3641_2.png) [@echoreply](https://boards.straightdope.com/u/echoreply)\ **Post date:** [July 14, 2021, 8:42pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/5 "2021-07-14T20:42:13Z")
My guess would be that relatives of your friend took a 23andMe DNA test, for example your friend’s cousins. The long-lost cousin found that some of those people were half-sibs and others cousins, and was able to determine that the uncle was the long-lost cousin’s father. Then contact was made through 23andMe between the long-lost cousin and your friend’s known cousins, and an exchange of names and other information happened. The long-lost cousin used this information to do other ancestry searching which turned up your friend. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [July 14, 2021, 8:42pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/6 "2021-07-14T20:42:40Z")
Perhaps a sibling of a friend has been tested. Or even a first cousin? ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [July 14, 2021, 8:58pm UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/7 "2021-07-14T20:58:18Z")
Yeah, my wife, who was adopted, was able to identify her biological father because one of his grandchildren submitted a DNA test. If you get a second cousin or closer DNA match, you can usually narrow it down to a very small list of candidates if you’re trying to identify a parent. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [July 15, 2021, 6:06am UTC](https://boards.straightdope.com/t/question-about-dna-testing/946490/8 "2021-07-15T06:06:55Z")
A gets a test and identifies who is his father, B. (C’s uncle). How? Presumably, either unless Uncle B is into 23-and-me (looking for all those residue of the times he sowed his wild oats?) . Or - one of Uncle B’s children (grandchildren?) call him D - took the test and A has already been in touch with that offspring to get the rundown on the family tree. I’m going to assume 23-and-me limits itself to giving estimates of “relatedness” and doesn’t collect information about random people who have not signed up for their services. So A has obviously talked to D who gave them the name of C and C’s father, B’s brother(?), filled A in on the whole family tree, etc. Or if Uncle B had no children, maybe it’s a further relation like another cousin or such that’s playing the part of D and being the contact telling A about the family. After all, if he needed to know “who was on shore leave from the USS Minnow in San Diego in 1988” even a grand-nephew might be able to answer that question for A if they know a bit of family history. Has A been making the rounds? Did he talk to any other known relatives? Also, always be wary that someone may have done a little bit of research and is running a scam - although unless C has a decent amount of money, it would seem to be a lot of effort for a tenuous payoff. And also, note the old mind-reader trick. By engaging in conversation with leading hints, the person may be gleaning even more information that they later in the conversations appear to already “know” if you forget you told them. (Isn’t that how the “your nephew is stuck in jail in Mexico and needs you to send him cash” scam works?) First thing I would ask is how (from whom?) he got C’s name and number.