When the DNA says your parents are related

One of the first things I do when helping someone with their DNA results is to check if their parents are related. This can explain unusual patterns of matches, for example, all seemingly from one side.

GEDmatch.com has a nice tool called “Are Your Parents Related” (AYPR) in the”Analyze Your Data” blue panel (middle right of page) which looks for places in the specified kit where the DNA is identical on both chromosome pairs, maternal and paternal. This happens when you inherit the same segment of DNA from each parent because they are related. We call this a homozygous run which is a fancy way of saying a stretch of identical DNA on both sides.

CeCe Moore specializes in helping people who make this discovery. Click here for the informational brochure she helped Brianne Kirkpatrick, genetic counselor, create. It includes where to get emotional support.

My goal is to help you figure out what the DNA means yourself. Can you deduce what the relationship of those parents is? Well a very simple rule of thumb is to multiply the shared DNA from AYPR tool by four and look up that new total at the DNA painter calculator for the possibilities. Then do further family DNA testing to confirm.

Why does this work? Let’s look at the numbers. Suppose your parents share 25% of their DNA. They will pass about half of that to you, so 12.5%. However only about half of that will be the same DNA so it will show up as about 6.25% on the AYPR tool.

Look at the image. The total is 215.3 when you multiply by 4 you get 861.2. You might look that up before you read on …

I was consulted on a case where the father was found to be not related via DNA and the secret family story was that the biological father was actually a cousin of the mother. First cousins share 12.5% of their DNA. They each pass about half of that, so 6.25%, to their child. About half of that half would be the same DNA from both parent, 3.125% or about 233 cM. The image of the matching chromosomes is shown above and the number summary is:
Largest segment = 46.7 cM
Total of segments > 7 cM = 215.3 cM

These numbers suggest that family story is true. Further family testing, for example of the child of the first cousin and suspected Dad, will confirm it.

First cousin marriages were quite common in the past and are legal in many states. Uncle/niece marriages were also common in some communities. People from very endogamous groups will often have distantly related parents and it may or may not show in this tool. My Jewish husband actually shares 13cM of X DNA with me and my aunts, probably from quite far back on my German Jewish grandad’s line and his Galician descended mother’s line.

Now to look at the more closely related cases. Please remember, if this is like your case, it is no reflection on you. If your parents are related your DNA is probably just fine. Plus it will have no effect on your own children’s DNA.

In another case, a woman contacted me and told me that the rumor was that her Dad was her mother’s half brother. Does that fit with the numbers below and the image to the right?:

Largest segment = 49.4 cM
Total of segments > 7 cM = 290.0 cM

Multiplying 290 by 4 you get 1160. While this number has not been observed by Blaine’s project for half siblings, it is theoretically in range.

Further testing of family members confirmed this rumor.

Then there was the case of the fellow whose father was also his mother’s father. CeCe Moore figured that one out. Click here for his write up of his story.

These are his numbers and his image is to the left:
Largest segment = 77.3 cM
Total of segments > 7 cM = 756.7 cM

Multiplying by 4 you get 3026.8 … which falls between the normal sibling range and a parent-child as expected  …

[UPDATE 26-Jul-2018 at 13:00: A parent passes half of what they got from each of their parents to a child thus about 25% of that child’s DNA is from each grandparent. The parent/grandparent passes the new child a different 50% than they gave the child/parent so the DNA would end up being about 18.5% identical (half of 25 plus half of 12.5). In the case of full siblings, since about half of their shared DNA is the same on both sides for them, the resulting identical DNA is not half of the shared half so 12.5%, but all of the 12.5% that was the same on both chromosomes plus half of the remaining 12.5% so more like 18.5%, thus it overlaps the other case]

How can you tell the difference between the child of full siblings and the child of a parent-child relationship other than emailing CeCe? Andrew Millard, whose simulations Leah Larkin often blogs about, came up with the idea of mapping the shared segments. If they are all from one side, then it is the parent-child case.

[UPDATE 27-Jul-2018: Leah Larkin has shared her analysis of a case where the mother was known and the question was whether the dad was her father or brother. She uses several of Andrew Millard’s simulations:
http://thednageek.com/gordon/ ]

The technical term for lots of shared segments between one’s parents is “high ROH” which sounds much better than the “in” word. If this is your case, please read the brochure above and find yourself some help and support. You are okay, really, it’s not your fault.

Remember multiplying by four just gives a rough estimate. The result is likely to fall in more than one range, so further family testing is best for sorting this out.


[Everyone whose data was shown here has given permission for this usage. No names or kit numbers are shown]





41 thoughts on “When the DNA says your parents are related

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  1. What a clear and easily understandable way to explain this. I have never seen an explanation on this. Thank you Kitty!!!! Now I understand how to read the numbers.

  2. I’m hoping this will allow me to clarify for an adoptee I’m helping. CeCe has already confirmed it’s either the BM father or brother. The adoptee does want to know the truth.
    I’ll be working on this later today.
    Thank you!

  3. An important comment by Kathy Johnston on FaceBook (who gave permission to publish here) somewhat rephrased:
    Many people expect to see these identical segments when their parents are closer than 4th cousins. Just because this comparison is negative does not mean they are NOT related within a genealogical time frame. … The AYPR tool cannot rule out distant and even some closer relationships. Even if the parents share more than one segment with each other, these matches may not get passed on to the child.

  4. Thanks for the clear explanation that shows how to determine the estimated relationship based on ROH. My grandparents were 1st cousins once removed. My dad and the two siblings that I’ve tested have ROH results that reflect that and the 1C1R is the estimated relationship with your method of calculating.

    I’ve always known about this and it was never a disturbing fact. At family reunions I remember hearing a relative say, “Oh, yes, Uncle Rob married cousin Erie.” It was said in the same manner as someone would say, “They make the most delicious pecan pie!” When I tell other people about it, they don’t think of it the same way, though.

    I tell you, though, it can be very confusing when dealing with DNA matches. Because not only were they 1C1R, they were also 2C1R and 2C2R in such a way that all – every single one of them – of my grandfather’s family lines end up in my grandmother’s family tree. They even have the same maternal haplogroup!

    I wrote a blog post about the entangled family lines: http://history.jciv.com/2016/09/entanglements/

  5. This is fascinating! It also makes me wonder if I can use my DNA analysis to confirm or disprove a discovery I made through genealogical research. It involves my father’s maternal grandparents. There are no rumors (at least none to my knowledge), but when I found my ggf’s immigration record, it said he was coming to the US to live with his cousin. I happened to discover that this cousin was my ggm’s uncle (her mother’s brother). I wonder whether he used “cousin” as a sort of euphemism on his immigration records, or if they truly were cousins. I haven’t yet been able to find an answer by using standard genealogical methods. Is there anything I can check for in my DNA that may help solve this mystery?

    • Unlikely you can prove anything via DNA that far back unless you can compare lots of descendants of his parents to descendants of her parents.
      Also cousin may have been a 2nd or 3rd or… Best to try and look I the records

  6. What type of DNA testing do you need to get the information that would reveal something like this. There’s so many tests and they get fairly expensive. I’m not interested in finding anyone.

  7. So fascinating! The possibilities are endless, huh?

    There is a rumor that my maternal 1st cousin (female) could be the daughter of my maternal full aunt and A) (hopefully) my cousin’s father, the man that raised her, B) our maternal full uncle (full brother-full sister incest), C) our maternal full grandfather (full father-full daughter incest). Gross, I know. My cousin, my mother and I have all done 23andMe.

    My cousins parents are deceased, as is our maternal grandfather, so they cannot be tested. The maternal uncle in suspect is alive, elderly and has no interest in genetic testing.

    I show up as a solid 50% identical to my mother across every chromosome, as well as with my father. My cousin shares 2127cm on 54 segments with my mother, which, according to ISOGG, puts them at the high end of being aunt-niece or half-siblings. On some of the chromosomes they are 90% half-identical. They share 126.87cMs on the X chromosome, ~70% of that half. However, my cousin and I share 0 of the X chromosome, yet I am a solid 100% identical on the X I got from my mom. How I can share 100% of an X chromosome with my mom, my maternal cousin shares ~70% of an X chromosome with mom (my cousin’s full aunt), yet my cousin and I share 0%? Explanations? Any input is helpful! Thank you

    • Hi Kiki,
      What does the “are your parents related” tool find on your cousin’s kit? That will tell you if there was incest involved.
      As to the X match, your mother has TWO X chromosomes that is what makes her female but she only passes ONE X to you, which is usually a recombination of her two Xs. It just happened that she passed you X that does not overlap with the X she shares with your cousin!

      • Thank you so much, Kitty. Got it, I think! So the X my mom passes to me is NOT an exact copy of one of her Xs, but a combination of her Xs? Where as the X that my father passes on to me IS an exact copy of his X? Unfortunately (or maybe fortunately) my cousin does not want to use the “are your parents related” tool. She’s healthy and happy, her offspring are healthy and happy. Will it really matter 50 years from now? Maybe it’s better to let sleeping dogs lie. I’m still super curious, though. 🙂

        • If her parents are related it does not mean she will have health issues and her children rate to be fine. It usually takes generations of inbreeding to cause serious problems. The issue is when you get two copies of a deleterious gene, you are stuck with it …
          You can run the AYPR tool on her kit number if you must but then you may be holding a secret which can be hard on you. So my advice is leave it alone; it’s up to her.

          • Yes, I agree. My thoughts exactly. I’m going to let this one go. Satisfying my morbid curiosity isn’t worth causing anyone heartache. Thank you for all of your information on this whole DNA thing. So fun and interesting!

  8. Yes a valuable tool and article!! I have 56.9cm on the gedmatch utility- & 56.9×4=227.6cm between my parents would make them around 2nd cousins or first cousins twice removed!

  9. I was adopted, and uploaded my Ancestry DNA results to the GenMAtch AYPR tool.

    I am upset by the results. Partly because I don’t completely understand them, and also because they show my biological parents were related. If I’m doing the calculations correctly, they were pretty close.

    I’m not sure what to do with this information. My own kids are all nearly adults now, and there are some significant health issues, as do I, and I am worried about them.

    • Mary Jane I am so sorry for your painful discovery. Do read the pamphlet linked to above and perhaps join an online support group.

      To slightly repeat what I said above about having closely related parents “does not mean you will have health issues and your children rate to be fine. It usually takes generations of inbreeding to cause serious problems. The issue is when you get two copies of a deleterious gene, you are stuck with it …”

      If you think your health issues are genetic please discuss this with your health care provider. Have you uploaded your results to Promethease.com ??? That will list the known health issues in your DNA but of course not everything has been figured out yet

  10. Would the GedMatch tool show if my Grandparents were related?

    My father was adopted and he’s also deceased. My sister and I have both taken DNA tests from Ancestry and have a pretty clear picture of who are ancestors are…we just need to narrow down who his parents are amongst the families we show relationships to.
    There are stories about a particular Uncle being very handsy (and going to jail for rape of a 14 yr old). We are wondering if it’s possible that this Uncle got his daughter pregnant resulting in our father.
    Basically trying to disprove every possibility at this point.

    • No the Gedmatch tool would only show that for your father’s DNA.
      However a one to one comparison between you and your sister would likely show higher shared DNA and more FIRs than expected for siblings.

  11. I’m so glad I found this and it looks like you are still answering comments. You said it takes generations of inbreeding to cause serious problems? I’m doing Ansetery DNA my results will be ready in a week.

    (As far as I can see about 4 generations past my paternal grandmother…half brother and sister married had a son and a daughter, son married outside of family daughter married a cousin maybe second (I’m only doing parents in my tree) the brother and sister each had a child with thier partner and those children married each other (first cousins) Then the first cousins had a daughter and married a son the half brother and sister had much later (great uncle?) They had a daughter who was my fathers mother….and rumor has it she may have married her first or second cousin but not positive on that.)
    If all of this is true and I’m pretty sure I am, I have checked birth and marrige records…will this show in my DNA results?
    I have 5 children 2 autistic one of them sever as of right now nonverbal, stereotypes…(hand flapping TICS possibly seizures etc) as well as had transpostion of the great Arteries when he was born. Another child speech delay but therapists say will catch up with a year of therapy and the other child has ADHD has bad teeth and will need braces in a few years.
    Hope I explained everything ok its late. Thank you for any info! If you have detailed information also please send to my email. As I don’t know how bad this is.

  12. Cara –
    In theory if you are OK and not married to a close relative these problems in your children are not from the inbreeding a few generations back. When your results come in, you need to upload them to GEDmatch Genesis to look in more detail. Also upload to promethease.com for health results in your ancestry test.

  13. Thanks for the article! It is very helpful. I have someone’s DNA test that I am helping with who was adopted and knows nothing.

    We have found a couple of distinct lines and are trying to get the relationships nailed down. One match came up as a half sibling.

    Comparing DNA from a half sibling and trying to piece together genealogy it appears that his parents would be 2nd cousins. Using the “are your parents related” tool does not show up any relation.

    Is that possible or would the tool show the match or sure if it was?

    I am willing to rethink the DNA defined relationships but don’t want to rely on this tool if it may not show more distant relations accurately.

    • It is barely possible that the AYPR tool would not show anything for that half sibling, but with parents of 2nd cousins I would expect to see about 50 cM there (one fourth of the average amount for 2nd cousins). So the half siblings parents are likely slightly more distanty related if related at all.

  14. This is a slightly different topic but I’ve not been able to find anywhere else to ask. My DNA and the DNA of the man I thought was my father was sent into Ancestry and we showed up with only 1689 matching cM’s across 59 segments. Is it impossible with these results that he’s my father? His sister which I believe is my aunt matched me with 1606 across 55. They had one other brother who is now deceased. Thank you for your help! Would it be possible for you to email me a response? Thanks so much!

  15. If I am doing my math correctly my 1143.7 segments x4 are 4574.8. It’s saying 1.6 number of generations apart. Is that more likely a father/daughter or brother/sister

    • Anna
      Sorry I did not see this sooner…see my response to Marcos below. Probably parent/child but I will respond to you privately as well.

  16. Largest segment = 91.9 cM
    Total of segments > 7 cM = 767.7 cM so x4 is 3070.8 i believe. so what does this mean?

    • This means your parents were siblings or possibly a parent and their child. I would need to analyze it some more to figure out which

      I understand how difficult this is. there are some resources listed in my blog post

      Here is an article Leah Larkin wrote about a similar situation

  17. My GedMatch says parents are distantly related. My ROH is 7.2 and 701 SNPs. I can see 4th cousins that match both my father’s mother’s side and my mother’s mother’s side. What could this mean? Thank you so much!

    • When you multiply 7.2 by 4 you get 28.8 which can be anything from 3rd cousins to 6th cousins or even further back, check that at the calculator yourself
      The matching may mean that if you go far enough back on your father’s mother’s side you will find an ancestor that is also on you mother’s mother’s line or it could just be coincidence (depending how many of those double matches you see) …

  18. Thank you for your time! Using GEDmatch’s AYPR tool, it did not show any relation, but then there was a link for David Pike’s analysis, which I used, but seemingly got different result from GedMatch. Pike’s result show the following:

    Chr 2-ROH of 292
    Chr 6-ROH of 224
    Chr 6-ROH of 590
    Chr 19-ROH of 215

    Chr X the ROH is 17,604
    Chr Y the ROH is 885

    It states that total ROH is 214 and 6.786% of the reported genome, so if I times that by 4, is this 1st cousins? I thought maybe such high ROH on the X & Y chromosomes just had something to do with being male, but I tested two other males and neither showed any ROH on the X & Y.

    There is a lot of other information listed that I don’t understand and Pike’s tool does not tell you if they are related or not.

    Thank you for your comments!

    • Sorry I missed this question. The david Pike report is in different units, SNPs not cMs so not comparable to the tool I discuss here.

      A total ROH of 6.8% is high, but not abnormal for an endogamous population or if there is pedigree collapse (e.g. grandparents who are 1st cousins). Most northern europeans I have looked at seem to run from 1-3%

  19. I used the AYPR tool on the archaic DNA kits in Gedmatch.

    With 4 of those kits the multiplying by four goes a bit strange because the range would then be between 4,800 cM and 12,500 cM!

    The results were:
    – Hinxton 3 : 1,255.6 cM, 29.1 cM largest segment
    – Hinxton 2 : 1,857.7 cM, 32.2 cM largest segment
    – Hinxton 5 : 2,698.3 cM, 54.1 cM largest segment
    – Kennewick : 3,135.4 cM, 84.4 cM largest segment

    Despite the differences in size are they all parent/child or perhaps siblings?

    There is already a lot said about the Kennewick Man also known as the Ancient One, the ancestor of the Native Americans but I could not find anything mentioned about his related parents.
    Since he was of above average age (around 40 years) when he died more than 8,000 years ago I guess the inbreeding did not cause any significant problems!

    I initially used the AYPR tool only on the Clovis and Kennewick kits while trying to establish whether the 0.9% Mesoamerican and Andean that my father had in Myheritage is just noise or legit.
    Especially since he is of Asian and European descent.

    So in GedMatch I compared him with the archaic kits of Clovis and Kennewick.
    When using the 0.5 cM he has :
    – a HIR of 134,6 cM, 99 shared segments, largest 4,1 cM with Clovis;
    – a HIR of 2,8 cM, 1 shared segment, largest 2,8 cM with Kennewick.

    So it looks like he and the Clovis child do indeed have an ancient common ancestor (Asian / Beringian).

    • I doubt that AYPR is accurate on these ancient genomes. The multiplier is used on the cMs, not on the SNPs.

      Plus comparing at such a low threshold is unlikely to produce valid results

      The mesoAmerican prediction at MyHeritage is likely noise or a misinterpretation of the Asian DNA

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