Finding maternal grandfather

Question

All names in the description below have been changed.

My mother is going through a crisis because she is afraid my grandfather is not her biological father. She has been diagnosed with type 2 diabetes. She is the only person in her family, including my grandfather, my grandmother, and my uncle, to have diabetes other than her paternal grandmother (my great-grandmother), so my mom just chalked it up to that. Well, recently, she found out that her 2nd cousin DJ has type 2 diabetes, too, and that it also stems from his paternal grandparents.

Let me set the scene. My maternal grandmother, Lynn, was 16 when she got pregnant with my mom. My maternal grandfather, Jim, was 25. They had a shotgun wedding and my mom was born 8 months later. She’s always known this.

My grandma Lynn had a huge family, and she ended up having a niece named Gina who was older than her (my grandma’s oldest sibling had Gina at around the same time my grandma was born). Gina ended up marrying a man named Carl, my grandpa Jim’s best friend. These are DJ’s parents.

Carl died young, probably when my mom was around 4. She’s always just known him as a close family friend married to her cousin, and DJ was his son who was technically her 2nd cousin, but she’s always treated him like a brother. They favor each other in a bit of a weird way, considering they’re only 2nd cousins. I think you know where I’m taking this.

Turns out that my grandma and grandpa were going through issues when they got married, he was super jealous, she got pregnant, they got married, separated when my mom was around 3, got back together when she was 5. Right after Carl died.

After that, Gina and my grandma had a huge falling out and didn’t speak for the rest of my grandma’s life. There was no explanation given for this. My grandma was mad because she felt like Gina mistreated Carl, always cheating and being mean to him.

Now my mom’s connecting the dots, and she’s afraid that Carl is her real father, not my grandpa. That would make her current full brother her half-brother, and her 2nd cousin DJ her half-brother as well. We’ve been comparing pictures of everyone for the past 3 days, but anyone with sense knows that’s not a good way to determine paternity.

She’s completely torn up about it. To the point where she’s not even sure if she wants to know the truth. My mom is 55, and my grandma, grandpa, and Carl are all dead. It’s just my mom, my uncle, DJ, and Gina left, but my mom hasn’t seen Gina since she was in diapers.

She may not want to know, but I really do. Of course, if she really doesn’t want to know, I’m resolved to keep the secret. I’m interested in learning about my own health risks, considering the entire reason why this crisis came about was because of type 2 diabetes. But out of respect for her, what I’m really wondering is this:

Is there any way for me to do a DNA test to determine my maternal grandfather without getting my mother involved?

I don’t want to hurt her any more than she already is.

Answer 1

If your mom got DNA from her brother and from her "cousin" DJ, then a DNA test could establish which of them share a father. Either your mother and uncle or your mother and DJ.

However, you'll need consent from all parties to do this and while your mother may not want to find out, your uncle and cousin DJ may well want to after you approach them.

Answer 2

As others have said, DNA testing for your relationships would not be justified by a concern for type 2 diabetes, as genetic tendency for that is controlled by many different genes and also genes are not a strong influence compared to life style characteristics.

So I take it your mother has a first cousin whose child DJ is therefore her 1C1R (first cousin once removed) and so your second cousin. You are wondering if in fact your mother and DJ have the same father. If so, she and DJ would be half-siblings (as well as 1C1R via the other side). If that were true, DJ would be your half-uncle (as well as 2C).

If you and DJ get DNA testing of the kind offered by 23andMe and AncestryDNA, it would be fairly likely to show whether or not he is your half-uncle as opposed to only your second cousin. The relatedness is measured in units of cM, and the ranges of those two relationships barely overlap, as shown in this Shared cM Project diagram. Most likely the relatedness comparison you would see would have a value that does not fall near that overlap range, thus telling you which is in fact the case.