Any genetic scientists around to help me understand the results of a sibling DNA test?

[SnotsGotTheBoobies]

My sister and I always grew up thinking we had the same dad. But a few weeks ago, my mums current boyfriend said my sister wasn’t my full sister. He quickly back tracked, but I couldn’t stop thinking about it, so I ordered a sibling DNA test to see if we are full sisters or half sisters.
The test they did came back as inconclusive, so they looked for more markers. That also came back as inconclusive.
They sent their findings in an email. I have been up all night trying to figure out how it all works and what it means, and I think:
for each row of DNA, mine has to match my sisters for us to have that DNA in common. There are 53 rows, and my sister and I share 14 pieces of DNA. 14 is 26% of 53, so we only share 26% of DNA?
I obviously may have this completely wrong. But if somebody can help me with what percentage of DNA my sister and I share, I’d be really grateful, as if my workings are right, then it looks like we are only half sisters doesn’t it? As we share closer to 25% than 50% DNA.

Can't you ask your Mum ?

I'd suggest you go on FB and join the DNA Detectives group - there will be lots of people who can help you with this on there.

You cannot tell from this test whether you are full or half sisters, this can only show that you share parentage in some way. The percentages in common are averages from a population level, not to be applied at this level. It is theoretically possible for siblings to share almost no DNA, with the half from your mum/dad just not matching up - essentially getting different halves.

If you were to involve your mum in the testing, you could see if you share DNA beyond what is shared with her - that would be more conclusive. However, it might be easier to have a conversation about it first.

I have asked her, she is denying it, but I’m not convinced I believe her.

catspeakfordummies thank you. That’s promising that full sisters may share barely any DNA. Unfortunately my mum definitely wouldn’t consent to DNA testing. We may be able to get our dad to do one of that may help?

cornishgem1975 thank you, I’ll have a look at that group.

Getting your Dad to do a test would answer the question basically definitely. Because he’ll either share half his DNA with each if you (not the same half, but half) or he won’t share much DNA with your sister if she is not his child.
But couldn’t you just ask your Dad about it? Or is this a secret that he doesn’t know about? As in, your mum was pregnant to someone else but told your Dad he was the father? It would be a shit thing to do to him to convince him to do a DNA test without discussing the fact that you’re worried he may not be your sister’s dad.
Be careful OP, this might have serious repercussions for your relationship (or your sister’s relationship) with your Dad.

This is bloody awful, I would wait until your mother and her boyfriend are together and have it out with the pair of them. Someone is lying somewhere 🙄

Your mum not wanting to be tested does sound suspicious. I think talk to your dad.

twoweeksislong we haven’t told our dad what has been said yet. We really didn’t want to upset him in case we are both his. That’s why we have kept quiet.
Dad did mention years ago that he thought there was something going on between our mum and the guy we think is the potential father of one of us when he was still married to her, but I don’t think it has even crossed his mind that one of us may not be his.
this is all so messed up. I just wish we had a definite answer. I just don’t want to tell all the family and cause a lot of drama for everybody in case we are full sisters.

If your dad/whole consider you two full sisters - and you have a good relationship with him - why risk hurting him?
What would it achieve?
I get the curiosity. But it’s not worth it, in my opinion.
Being a parent goes beyond just genetics.

(if you really can’t help yourself - I’d do it discretely. But then again - what would you do with the information? Suppose dad isn’t your dad. Or even not a dad to either of you. Then what?)

I’m no scientist but my understanding is that you share 50% of dna with your full siblings, so I’d guess that you’re half sisters based on that result.

There's no definitive answer without your Dad. Would he potentially be interested in looking for ancestors/relatives as some of the sites do? You could ask your mum and dad to give DNA for that. My FIL got really into tracing his ancestors when he retired.

I really don’t think there’s anything to be gained from pursuing this beyond you and your sister. Keep it between you two, otherwise it could really poison family relationships and be very upsetting for your dad, and possibly your mum too.

The boffins of the internet will give you answers for sure, people are obsessed with this stuff.

Have you not contacted assuredna to ask them for some input on this? Have they explained what they mean by inconclusive - what was inconclusive - what was the question? Are we full sisters? Is that the question that they said they were unable to answer or was there something wrong with your sample or the processing of it? Or is their testing quite limited/outside the scope of what they do? I've never heard of assuredna - why did you go with them? Why not one of the more well known sites like Ancestry or 23andme?

@larkstar yes 23andme are very happy to pronounce people as half siblings.

And I’m thinking that “inconclusive” is probably an answer they give rather than saying that you are definitely NOT full siblings, because such information can have legal ramifications. Some of these genetic testing companies have disclaimers in the small print about the use of the results as legal evidence.

Could you and your sister do an ancestry DNA test and see if you share matches on your Dad's side of the family?

Sometimes it is best to let sleeping dogs lie, OP. If you love your sister surely none of this matters? Or is this an act against your mother?

The average amount of DNA shared between full siblings is 50% if you look at the population as a whole. That doesn't mean that all full siblings share 50% of their DNA. In theory, you get 50% of your DNA from each parent, so one sibling could have entirely different DNA from their sibling if each sibling got the 50% that their sibling didn't get IYSWIM?

There are other factors that will affect the results:

1. The tests aren't 100% accurate
2. There are different ways that the matches are counted when they tell you the % of DNA shared: www.thetech.org/ask-a-geneticist/dna-shared-siblings
3. I can't read the photo but I am guessing that you had identical DNA at 14/53 sites (loci) of the DNA sites that were tested. The test does not test all your DNA, just a few locations. They looked at 53 loci out of many thousands... So you share 26% of the DNA that they tested.

The only way that you can be sure whether you share a father is for your DF to be tested too.

I wasn’t really aware that the ancestry sites could test siblings. I just googled sibling DNA test and assure DNA was one of the top ones with good feedback so I went with them.

I understand what you’re saying about letting sleeping dogs lie, but I just feel so deceived and I want to know whether I’ve been lied to my whole life. I don’t think I can pretend everything is ok with my mum when I’m still unsure about all this.

Your mum's boyfriend is a piece of shit. Buy three 23andme or Ancestry DNA test kits. Give one to your dad. Say you're exploring your ancestry and need his to get your paternal haplogroup. With Ancestry, you can manage all 3 profiles under one account. With 23andme, I think you have to set up separate accounts. Either way, it will reveal the truth. What you do after that you need to think very carefully about.

The other way to know for sure is if you know the man who is the potential father to your sister and he would be willing to have the conversation and do the test. There’s one caveat here. If he is her father, the test would confirm that pretty definitively. If he isn’t her father, it wouldn’t really confirm that your dad is actually your sister’s biological father.

I would get two 23andme kits: one for you and one for your sister. If you just share a mum you'd have the same mitochondrial DNA, and on autosomal DNA you'd share approx 25%. You may or may not match on the x chromosome.

If you also share a dad you'd be expected to share approx 50% of DNA including a full match on one of your x chromosomes, as your dad has only one X to "give". You could also ask your dad you do a test, but that would be a third one in addition to the above.

You get 50% from each parent but each sex cell is 50 /50 of the parents 100%.

Hence why a genetic condition has 50% chance of being passed on. You need the other puzzle piece.

But I agree be careful. You might get a answer but what will it give you? I have some dealings with genetics and whatever councilling and scientific knowledge i have ( bio degree) it didnt come close to help dealing with what I do with that info.