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]





19 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!

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