If your uncle could also be your half-brother, is there no point?

March 17, 2015 8:21 PM

I have a family member who I've been told is my uncle, but could very well be my half-brother (almost 50 years ago, my father slept with his mother, who was married to my father's father at the time). I really want to know what the truth is: is this relative my uncle, or my half-brother?

Looking up genetic relatedness online, it looks like uncle-niece relationships share the same amount of DNA as half-sibling relationships (both share 25% of the same genetic material). I've ordered a siblingship DNA test, and I talked to one of their specialists over the phone, but... I don't feel reassured. I feel like the specialist told me "yes" to my questions and whether the test is accurate or not, it'll good-bye $300+ for me.

Basically I feel like the DNA test could say "Yes, you two are half-siblings" when basically it's just picking up on the degree of relatedness between an aunt/uncle and a niece/nephew.

My question is: Has anyone here ever tried DNA testing under similar circumstances? Did you have confidence in the accuracy of your results? Were you satisfied that you knew the person was your aunt/uncle (and not your half-sibling)? Thanks!

posted by human ecologist to Science & Nature (23 answers total)

Hang on, your father slept with this uncles mother or with his own mother?
posted by taff at 8:32 PM on March 17, 2015 [4 favorites]

Response by poster: My father slept with the uncle's mother.
posted by human ecologist at 8:35 PM on March 17, 2015

FWIW, a friend did 23andme (which is considerably less than $300) with some of his relatives, and the reports they got back concluded that his half-sibling was his grandmother, and his nephew (by his half-sibling) was his first cousin. (His half-sibling is definitely not his grandmother and his nephew is definitely not his first cousin.) So if it's really important for you to know and you have everyone's cooperation, I'd encourage you all to pay a premium to go through an actual genetic counselor who can sort this out and explain limitations and interpret results, and not try to do this yourself with a direct to consumer kit.
posted by blue suede stockings at 8:43 PM on March 17, 2015 [8 favorites]

Test if he's related to your father's mother. If he is, he's your half-brother. If he's not, he's your uncle.

If she's dead, this might be tough. Does she have any sisters?
posted by amaire at 8:45 PM on March 17, 2015 [7 favorites]

I agree with blue suede stocking that this is best coordinated though a GC who can help assess which family members are most appropriate and then help sort any resultant questions/fallout. Also, all family members should be properly consented as to the purpose of this.
posted by beaning at 9:01 PM on March 17, 2015 [3 favorites]

Just to clarify, your father slept with his stepmother? So this person could be his half brother, or his son and your half brother?
posted by alms at 9:15 PM on March 17, 2015

Just trying to understand...

Your father Alex is the son of Bob and Charlene. At some point later, Bob married Debra, and Debra had a son Earl. And you're wondering whether Earl is the son of Debra and Bob or Debra and Alex.
posted by ROU_Xenophobe at 9:15 PM on March 17, 2015 [2 favorites]

I'm a layman, but if possible, it sounds like the best way to do this is to get DNA from your uncle, your father, and your father's father. That will be an easier test than testing you and comparing it to your uncle's.

(On preview, Chocolate pickle makes a good point)
posted by tinymegalo at 9:31 PM on March 17, 2015

Response by poster: @ROU_Xenophobe: Correct. Earl and I have both been told that Earl is the son of Bob and Debra, and Earl and I want to know if he is really the son of Alex and Debra.

Earl and I are both adults, consenting to have a test done via mail-in buccal swabs. The family members older than us are all passed on from various natural causes. It's really not for posters to project the value of this for us, but if it helps, Earl and I would just like to enjoy more truth in our lives, and be able to speak honestly with each other about how we know each other. We find great healing in laughing and joking about how sick and sordid this family tree is, either way. It's not for anyone but us to decide why this is worth knowing.

Please I'd appreciate only helpful answers or inquiries that specifically address the question posed. Thanks.
posted by human ecologist at 9:36 PM on March 17, 2015 [15 favorites]

uncle-niece relationships share the same amount of DNA as half-sibling relationships (both share 25% of the same genetic material).

So, this means the test won't give you useful information, and that you'll have to rely on old-school sleuthing.
posted by cotton dress sock at 9:41 PM on March 17, 2015

Unless there's a qualitative marker that denotes particular kinship relations (my woefully underdeveloped knowledge of biology leads me to think, "probably not").
posted by cotton dress sock at 9:43 PM on March 17, 2015

Aha. So you want to know if your dad slept with his step-mother to create the man you know of as a half brother to your dad...ie your (half) uncle.

I can't help with the current state of play with genetics (my knowledge is old and basic), but I think I've straightened out the question.
posted by taff at 9:52 PM on March 17, 2015

I'm wracking my brains and I can't come up with a way to do this with just samples from you and your uncle/step-sibling. I think you need a third sample to triangulate; this also definitely sounds like a case for a genetic counsellor as the inter-relatedness makes it way more complicated than the usual 'who's the father' questions companies like 23andme can (probably) deal with.
Sorry.
posted by AFII at 10:01 PM on March 17, 2015 [2 favorites]

You need to find out if he is related to Charlene. I'm not sure how you do that, but that's your angle. I think you've implied Charlene is dead, but does she have any living relatives who are not also related to Bob?
posted by If only I had a penguin... at 10:05 PM on March 17, 2015 [4 favorites]

I don't know if this helps but it seems like Bob is the common denominator. I agree this is a complicated problem and you should see a genetic counselor to know for sure and have peace of mind.


This part we know for sure:

Bob -- Charlene
     |
   Alex -- Mary
          |
      Human Ecologist (1/4 Bob, 1/4 Charlene, 1/2 Alex, 1/2 Mary) 

If Bob is Earl's father:

Bob --- Debra 
     |      
   Earl (1/2 Bob instead of 1/4 Bob and no Charlene, Mary, or Alex but Alex might share some of that 50%)

or if Alex is Earl's father:

 Bob --- Charlene 
      |
   Alex --- Debra
           |
         Earl (same 1/4 Bob, 1/4 Charlene and 1/2 Alex as you but Debra instead of Mary)

posted by bleep at 10:20 PM on March 17, 2015 [1 favorite]

There's only a small chance things would even line up such that this could help you figure this out, but do you know or could you find out Alex and Bob and Debra's blood types?
posted by needs more cowbell at 12:07 AM on March 18, 2015 [1 favorite]

(For example: if Debra is O+, Alex is AB+, and Bob is B+, if Earl is A+ you'd know he has to be Alex's son. There are a few other ways this could work. )
posted by needs more cowbell at 12:22 AM on March 18, 2015

If your uncle has any siblings whom you're more confident are biologically your grandfather's, testing them could give you an answer.
posted by metasarah at 6:25 AM on March 18, 2015

I agree that you need to find someone living who is related to your grandmother (Alex's mom) but not blood related to Bob or Alex and test their DNA. You should be able to do this as part of the siblingship test you already ordered.
posted by muddgirl at 6:30 AM on March 18, 2015 [1 favorite]

Also, more specifically related to your question - I don't know what the $300 test encompasses but a lot of amateur family sleuths use $99 autosomal DNA tests from 23andme (or Ancestry.com or family tree DNA), and autosomal DNA matching software like gedmatch to trace down family mysteries. It's basically a DIY approach to what sibling matching companies do. There are also independent genetic counsellors that can help interpret data (for a fee). If you look at the cost of independent tests + an independent counsellor, $300 isn't exhorbitant.
posted by muddgirl at 8:11 AM on March 18, 2015

Looking up genetic relatedness online, it looks like uncle-niece relationships share the same amount of DNA as half-sibling relationships (both share 25% of the same genetic material).

But if Earl is your "uncle" (Bob's son), then he'd really only be a "half-uncle," i.e., one of Earl's parents is one of your grandparents. I think the uncle-niece relationships you're seeing are the conventional ones, i.e., both of the uncle's parents are two of the niece's grandparents, which would be 25%. But a half-uncle would only share 12.5% of his genetic material with you, so a test of just you and Earl should be able to distinguish between those two possibilities.

I think. I'd check with a professional to confirm that analysis. I am not that professional.
posted by DevilsAdvocate at 10:18 AM on March 18, 2015 [4 favorites]

I'm guessing you've thought through family heirlooms / junk that might have Charlene DNA on them - hair follicles (hairbrush), blood stained clothes/cuticle trimmer/etc., old toothbrush? This is getting into forensic techniques that a standard genetic counselor might not be up on, though.

If Charlene had or carried an unusual genetic condition, you could have Earl tested to see if he carries it as well. Probably talk to a genetic counselor if you go down that route, though, lots of textbooks purport that a bunch of easily-measured traits are single-locus dominant/recessive for the purpose of easy in-class exercises when the truth is more complicated.
posted by momus_window at 12:53 PM on March 18, 2015 [1 favorite]

Best answer: It seems to me this is correct:

But if Earl is your "uncle" (Bob's son), then he'd really only be a "half-uncle," ... But a half-uncle would only share 12.5% of his genetic material with you, so a test of just you and Earl should be able to distinguish between those two possibilities.

So if he shares 25% of your genes, he is a half-sibling and you have the same father. If he shares 12.5%, he is your half-uncle and you have the same grandfather. So all you need is a test that distinguishes that much.

This chart agrees that a half uncle only shares 12.5% of your DNA. It is listed in the same box that says "great grandparent."
posted by Michele in California at 2:31 PM on March 18, 2015 [1 favorite]