DNA to determine if sister share father

[Liz_in_Sussex]

Hi,

I have read through lots of posts on here and read articles on the ISOGG site but am still not 100% clear and am hoping someone could answer a couple of questions please.

If two sisters take an autosomal test and the results show them as sharing ~50% dna then they must share both biological parents.  If they only share ~25% then they must be half sisters and share only either their mother or father.

I think I am OK so far? 

But, is there any way of knowing which parent (both died a while ago) they share?   

My friend (who has no other siblings) was told a few years ago of a rumour that she and her sister did not share a father and they have decided they would now like to know the truth.

Would I be right in saying that if the test came back with only 25% shared dna then unless they found a paternal cousin to also be tested they may never know which of them was not their "father's" daughter.

Thank you very much,

Liz

PS I have warned my friend to think very carefully before going ahead with this 

[DevonCruwys]

Why don't they do an mtDNA test in addition to the Family Finder test? If they only share 25% of their DNA in common but have matching mtDNA results then they will know that they share the same mother.

You wouldn't be able to tell which of the two daughters was the real daughter of the father without testing other relations of the father.

You are right to warn your friend to be very careful. There's always a risk that they could get an unexpected answer such as that they are not sisters at all.

[Liz_in_Sussex]

Thank you - I hadn't thought of that.  I'm hoping they won't go ahead with it (no smoke without fire and all that) but will explain the mtDNA option if they do decide to.

Liz