Genetic testing: any way to tell the difference between mother/daughter and sister/sister?

I have a suspicion about the parentage of a close family member. Her older sister was 16 when she was born, her mother “hid” the pregnancy and surprised everyone with the new baby, and the older sister’s later husband and mother-in-law treated her very badly for some unspoken reason.

I believe that older sister may very well have been this relative’s mother, and my relative was raised by her grandmother.

Knowing that the mitochondrial DNA will be the same, is there any way to tell, looking at both women’s genome’s if they are sisters or if they are really mother and daughter?

A mother and daughter will have exactly 50% of their DNA in common. Sisters, whether by the same father or not, COULD by chance have exactly 50% of their DNA in common but very very very likely will have some other varying amount. I’m not sure how calculating the percentages is done when comparing tested DNA, but that would be the way to do it.

ETA: I should add the caveat that the “exactly 50%” assumes that mom and dad aren’t closely related.

Also, beyond the 50% measure, we can rule out a mother-daughter relationship with a single gene.

e.g. Woman 1 has genotype BB, Woman 2 has genotype bb.

We can say that either they are not mother daughter, or there has been a highly unlikely mutation (a few example of genes like this and the mutation possibility can be effectively ruled out).

Yes, its easily possible (for the reasons stated above). You just need to look at enough markers - the usual number used in DNA fingerprinting would be enough.

Would it be easier to tell with a comparison of the grandmother/granddaughter genes? That would be either 25% match or 50% match. You don’t say, but I could imagine various reasons that it would be impossible, or at least very difficult, to make that test.

I asked a similar question in this thread a couple years ago.

At any locus, a mother and daughter will share exactly one allele. Siblings will share one allele on average, but may share 0, 1 or 2, in a 1:2:1 ratio. It’s a bit more complicated because even randomly chosen people share some alleles. Mijin’s example is a case of sharing 0.

Better test “dad” or (granddad), mum (or grandma) and daughter (or granddaughter).

You test the “parents” and the “child”-if 50% of her DNA appears to have come from them, but 50% appears to come from a third source (or there are too many dissimilarities to be explained by spontaneous mutations alone)- bingo she’s their grandchild.

Well, the other detail that makes me very suspicious is that close relative has light brown eyes. Her “parents” both have light blue eyes. I know there are more complications to eye color genetics than what’s taught in high school, but dysfunctional family secrets seem far more likely.

How would you get the DNA? Surely the older sister knows the truth, so how would you get her DNA to test?

Bingo! This is one of the biggest giveaways that someone’s parentage is not what it is alleged to be. The other biggie is genetically-impossible blood types, ie two O-positive parents having an A-positive child.

Errmm… naive question: since it’s no longer the 1940s/50s/whatever wouldn’t politely asking big sister be an option? Though I guess if the family have (potentially) gone to those lengths to conceal what happened then just asking might not be an option.

Families are odd things – I have a sister-in-law and three cousins who were adoptees and who knew from an early age, and a step-sister who was adopted and never told, only finding out after her adopted mother died from a disease that would have had implications for her had she been genetically related. she got to find out that not only was she adopted but that she had three full siblings that she never knew about.

Also reminds me of a little incident where my wife and I (brown/brown hair and brown/hazel eyes respectively) had our blonde and blue-eyed son at a party when he was little. A friend looked at him, and at us, and started to say something like: “Hey, how come if you’ve both got brown hair…”, before turning a lovely shade of pink and shutting up fast. :slight_smile: (What she didn’t know is that both our families have lots of red hair and blue eyes – we both just got the brown, but son picked up the recessives).

“Older sister” has severe Alzheimer’s and is currently under 24 hour care. She’s also half a country away. There is a second sister (14 when my relative was born) and a brother (10 when my relative was born), but I suspect that if they know, it is simply Not Spoken Of.

It’s also possible that relative’s “mother” and “sister” managed to hide the pregnancy so well that family members weren’t aware. Everyone was, apparently, genuinely surprised at the birth. “Older sister”, before Alzheimer’s set in, put up a very positive face to her family, but as things have fallen out, her relationships with husband and only child (?) had a lot of dysfunction. As in, the year her husband died, her Christmas letter was six letters of newsy chat and the third to last line was “Dear Hubbie passed away three months ago. Hope you’re all well!”

I’m no expert, but aren’t the genes for blue eyes recessive? Meaning that it it’s a virtual impossibilty for two people with blue eyes to be the parents of someone with brown eyes?

Asking second sister or brother the blunt question, with a side helping of every-child-deserves-to-know-the-truth-and-if-you-don’t-speak-up-it-will-die-with-you-and-how-is-that-fair-to-her might get you an admission, particularly if this secret has been weighing on their conscience for decades. As to DNA testing with a swab from eldest sister, I think you’d need a court order because it sounds like she’d be unable to consent to it. Will a lab accept two samples and test them without documentation of any kind?

Otherwise… the reading of the Will might prove interesting when that sad day comes along. Oldest sister may take the opportunity to record the truth in her Will, or may leave an extra share to youngest sister.

Just speaking as someone who has an interest in family history and therefore has been privy to so many of the family secrets and scandals over the years, it seems to me that a lot of relatives do tend to unburden themselves of family secrets like this when the main participants die. Again, when eldest sister passes on, second sister or brother might think it is time to reveal the truth.

Out of curiosity, who does her birth certificate record as her parents?

Missed the edit window, but wanted to add:
If I was going to have the conversation with second sister or brother, I’d probably lead with something about it being funny that relative is the only brown-eyed member of the family, and seeing how they reacted to that. But I do honestly believe that asking a direct question (particularly if you can assume an air of “I already know the truth and I just want you to confirm it”) is the most effective way of getting someone to part with an old family secret.

I don’t think it’s as simple as that. My parents have apparently blue eyes, but there are tiny bits of brown in them if you look closely, same with mine. My sister has apparently brown eyes, but there are tiny bits of blue if you look closely. The simplistic brown/blue genetic dichotomy also doesn’t seem to account for the many different shades of blue, brown, grey, green, and even yellow that I’ve seen. My daughter has clear, solid bright blue eyes while my sister’s son has clear, solid steel-grey eyes. Apparently my daughter’s eyes exactly matched my grandmother’s in colour, dunno where my nephew’s grey came from.