# 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 …

# An unusual relationship shown in DNA

Sometimes DNA can reveal an ugly truth. A reader, let’s call her Patty, asked me for help explaining the unusual looking comparison of her DNA test results with her uncle Bob’s results. The surprising thing was the large number of green bars that she saw in the GEDmatch one to one comparison, indicating fully identical segments (FIRs), almost as many as a full sibling would have. How could that be?

Of course my first thought was that Bob is actually her full brother, that her mom, Janet, had a child out of wedlock who was raised by the child’s grandparents, Mona and Dick, as their son. This has happened in many a family. But that was not the backstory. Janet was a small child when her brother Bob was born. Bob and Patty also share just one segment of 27 cM on the X chromosome, which, of course, would be normal for a maternal uncle but low for a brother. Have a look at the comparison image from GEDmatch for chromosomes 1-22:

Thoughts? Usually only full siblings or double first cousins will have numerous fully identical segments, so what could this be? Obviously Patty’s dad would also have to be a close relative of Bob’s for there to be so many FIRs. A full sibling would usually show even more of them, however.