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:
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.
[UPDATE 3-AUG-2023] There is a calculator at SEG-CM for AYPR now which seems pretty accurate. Here is the link https://dna-sci.com/tools/aypr-cim/
[Everyone whose data was shown here has given permission for this usage. No names or kit numbers are shown]