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I was adopted and have identified my birth mother. I have 2 half-siblings though her—a brother and a sister. It turns out that my father is either the same as their father OR his brother.

Would my half-brother of half-sister offer a better DNA comparison if I want to figure this out?

In other words, we are possibly full siblings, or we are half-siblings AND cousins at the same time.

And would the difference in shared percentage of DNA be significant enough to differentiate? (i.e. 50% vs 25+12.5=37.5%)

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The answer to your question is: Yes. By that I mean, test them both!

If you use autosomal DNA testing, the gender of the person who matches you makes no difference (with one exception). It will not affect the total cM or the size of the segments.

Because you are potentially 3/4 siblings, you are looking for numbers at the top of the half sibling range and the bottom of the sibling range. But half and full sibling cM ranges overlap and regular half/full siblings end up there all the time. You might be able to rule out 3/4 sibling with an outlier result on the other end of the range, but it's not something you can just tell.

The exception to gender mattering in autosomal testing is the X chromosome. People vary in ways that defy textbook definitions but, in general, women have 2 X's, one from mom and one from dad. Men have only one and it's from mom. The X's do not match their parents perfectly (except in rare cases) because, like with the other chromosomes, they recombine before forming egg or sperm.

Unfortunately, looking at X results won't tell you if your birth dad is their father or uncle, unless you have DNA from the father or uncle (and if you had that, you wouldn't be asking this question).

Another DNA test you can do is Y-DNA. Only males can use this test because it requires a Y chromosome. The Y is passed down intact from father to son. Except every so often, there is a mutation. The problem is, if you have no mutations different from your brother, you don't know if that means you have the same father because two brothers (the father and uncle) are likely to match Y with no mutations. And if you did have a mutation with your brother, that wouldn't mean you were half-siblings because the mutation could just be in you (or him).

Go ahead and run the tests, with them both if you can. I recommend a genetic genealogy testing company. You might get lucky and be able to show or deny 3/4 sibling status with one of them. Chances are though that it will be ambiguous. You will have to wait to see if the uncle had any children who are willing to test.

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    The X that a child receives from the mother is likely to have recombined. The X that a child receives from the father has no other X to recombine with. As I understand it, a father's X is passed along unchanged (but won't match the maternal grandmother's X). See genetics.thetech.org/ask-a-geneticist/which-x-chromosome-mom – Jan Murphy Mar 19 '19 at 18:06

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