Baby daddy or baby granddaddy?
June 29, 2016 2:04 PM   Subscribe

Can genetic links in this particular scenario be diferentiated?

My new roomate likes watching reality TV, and we watched a few episodes of hispanic courtroom show Caso Cerrado together. Most of the participants sound like actors. Apparently the producers of the show have claimed that sometimes they reenact real cases using actors when the actual protagonists can't or don't want to appear on the show. For the older episodes that seemed plausible, the situations shown could have happened. But on the current season they seem to have gone all the way into fiction, I watched three different cases and there were several premises that I happened to know were false, but I'm curious about a particular case. I know the case is not real, I'm just curious about the plausibility.

For ease of visualization of generations let's say we have Abe, his children Barry and Beverly, and Beverly's daughter Connie.

Connie's father is unknown. Barry claims he accidentally read Beverly's emails and read one where she mentioned Abe had raped her years ago and he was actually Connie's father.

Are you following me?

Beverly denies this, and Barry says he had actually obtained DNA samples from Connie, had them tested against his own (yeah, this is probably illegal without the mother's consent, just suspend your disbelief for a moment, please), and Connie is his half sister, so she must be Abe's daughter (and we should probably rename her Bonnie).

So, is it really possible to determine via DNA testing wether Barry is not just Connie's uncle but also her half brother?

And if instead of Barry's we somehow had a sample of Abe's DNA, is it possible to tell Connie is not just his granddaughter but also his daughter?
posted by Promethea to Science & Nature (2 answers total) 1 user marked this as a favorite
 
Here's a good primer on shared DNA percentages.

If Barry's test indicated Connie was a half-sibling, that means they share about 25% of their DNA. This is the exact same amount you'd expect to be shared if Barry and Connie were uncle and niece. Barry's conclusion that their shared DNA indicates half-sibling is invalid -- it could just as easily mean uncle and niece.

If Abe is the father, Connie and Barry would share more DNA than a sibling or half-sibling -- they would share more than 50% of their DNA.

If Connie and Barry shared about 25% of their DNA (which is what would show up as "half sibling" or "uncle and niece", then Abe is... NOT THE FATHER! (insert dramatic reaction here)
posted by erst at 2:15 PM on June 29, 2016 [3 favorites]


Yes.

Assuming Beverly and Barry are full siblings:

If Abe is Connie's maternal grandfather only, Connie has 1/4 of Abe's DNA, and 1/4 of the Bs' mother's (call her Debbie) DNA. Barry has 1/2 of Abe's DNA and 1/2 of Debbie's DNA. The common contribution from each ancestor is the product of the fractions. Connie and Barry will have 1/4 * 1/2 = 1/8 of their DNA in common from Abe, and 1/4 * 1/2 = 1/8 of their DNA in common from Debbie, for a total of 1/8 + 1/8 = 1/4 of their DNA in common.

If Abe is Connie's maternal grandfather and her father, she carries a whopping 3/4 of Abe's DNA, and 1/4 of Debbie's. The common DNA between Barry and Connie will be (3/4 * 1/2) + (1/4 * 1/2) = 1/2.

If, instead, Beverly and Barry have different (unrelated) mothers, there is no common contribution from Debbie #1 and Debbie #2. Barry will share 1/8 of his DNA with Connie if he is her half-uncle only, but 3/8 if he is both her half-uncle and her half-brother.

It's even easier if you have Abe's DNA: Abe and Connie have 1/4 of their DNA in common if he is her maternal grandfather only, but 3/4 if he is her maternal grandfather and her father.

(When I say something like "3/4 of their DNA in common" that should be taken as shorthand for "3/4 of the DNA which is likely to differ between random unrelated humans" which is actually 3/4 of, very roughly, 0.1% of their total DNA. But given billions of base pairs, that's still easy to determine.)
posted by DevilsAdvocate at 2:23 PM on June 29, 2016 [1 favorite]


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