Need help confirming % of shared DNA

[OwlHoots]

I suspect that my grandmother who married and had a daughter(Jane) had an affair with her half brother whom she shared a mother with and produced another daughter(Mary). Could someone help me work out what the cms or shared percentage of DNA would be between these sisters and their own children?

Jane married and had a daughter(Sarah) and Mary married and had a daughter(Alice). Sarah is showing DNA matches with both grandparents but Alice is only showing matches with her grandmother and they're closer than Sarah's. I'd just like to confirm if this hypothesis could explain it. Thanks

[Biggles50]

Could be that the Father was not who you think them to be but we do not have enough info to be clearer.

How much cM does Sarah and Alice share with each other?

They should be 1/2 First Cousins (ignoring Pedigree collapse) with about 449cM shared between them, but this is the Mean figure and DNA Painter reports the range of possibles to be 156-979cM.

Listing the highest cM of a few matches each of them have could help us advise further.

[rebeccaclaire86]

https://dnapainter.com/tools/sharedcmv4 The DNA painter tool is good for that, alternatively there’s a calculator on MyHeritage that does the same, I’ve found DNA painter better though.

[SouthseaSteel]

Wow!!!

To focus on the OP's explicit questions on their suspicions, whether they are the right explanation of things or not, requires you to draw out a schematic tree of the parties concerned and to work out the MULTIPLE relations between Person A and Person B.  You then merely add the expected shared DNA amounts for the multiple relationships.

So say we are looking at one of the children Alice and one of the sisters Jane - which is the relationship the OP is interested in, then one relationship between them is half neice/half aunt.  This infers a shared DNA of 1,750 cM avg with range 1,349 - 2,175 cM. However, there is also another relationship between them which I THINK (please check as I have only drawn it out in my head and not on paper) is half cousin once removed. So they will share an additional 226 cM avg with range 57 - 530.

These two amounts and ranges (on a low and high case) basis are then added. So you could be easily be looking at 2,000 cM plus betwixt Alice and Jane.

Like I say please check the logic of these relationships!!!  And of course, as already noted other explanations may exist but good luck!!

[OwlHoots]

Thanks so much for your replies. There is 345cM shared between the two granddaughters Sarah and Alice.

The grandson of Jane's half brother from his own marriage has 361cM shared with Alice and 273cM with Sarah.

The reason I'm a little suspicious is because my grandmother never knew who her biological parents were, it was a big family secret and we only found out the truth after she died. It's also strange that Alices DNA matches can ALL be linked to her paternal side or her maternal grandmother. I know it's not impossible but it's odd that not a single descendent of her grandfather or distant relative has done an ancestry DNA test.

[SouthseaSteel]

I agree with your suspicions though a hypothesis supported only by lack of evidence is deeply unsatisfactory!!

Also, within your hypothesis, I think Alice and Sarah are half 1st cousins and also half 2nd cousins, which would take their average shared cM to 570 cM.  However, the range for H2C (9 -397 cM) is such that the additional relationship cannot be conclusively proven one way or the other.

[OwlHoots]

Thankyou so much for your help. It's frustrating that so many people have passed now we'll likely never know the truth. I'll keep checking and hopefully a DNA match comes up in the future that clarifies everything.

[SouthseaSteel]

True, but it only takes one person getting a test done to prove your hypothesis wrong which would be some kind of step forward

[Biggles50]

354cM shared between Sarah and Alice is right there for them to be 1/2 First Cousins.

Including the Grandson of Jane’s 1/2 Brother in the mix and if we estimate that his Mother would also have been a 1/2 First Cousin with both Sarah and Alice and would perhaps share about 500cM with each of them.

Based on these figures the hypothesis is in my mind not likely.

Yes there may be a degree of Pedigree Collapse but at present that may be a generation further back.

As is often the case DNA is a waiting game and hopefully in time more tests will reveal themselves.