Ancestry DNA says my youngest sister may be a half sibling

[ancestrydnashock]

I got my updated Ancestry DNA results and only have 26% dna in common with my youngest sister. We are of multiple ethnicities; my dad is 100% X while my mum is multi ethnic. Mum passed away two years ago and dad died decades ago. I look like my father, while she takes after our mum, so no cause to think anything odd.

I'm didn't think much about the results at first, but then did a google search which said, while very unusual, it was possible for a full sibling to show up as a half sibling. I then asked AI and input more info, including the name of the test, then got the answer that the shared cM amount, indicated a half sibling or niece.

The only thing I had found mildly questionable before, was that my sister had 40% and not 50% X dna in her test results and that her dna was from a different part of the country, (think east, not south like mine).

To say I'm shocked is putting it mildly and I have to think about what, if anything, I do next.

I'm the image of my dad's sister and all my life have been told how much I take after him. I recently showed a new friend his photo and she even commented on the resemblance. The known half sibling is from my mother's first marriage.

Agree, and children don't always inherit the same 50% though, so it is quite common for siblings to inherit different traits from one another, especially with multiple parental ethnicities. One of my relatives has inherited a very unusual and specific thing common in the dna from populations in another part of the world, whereas their siblings have not.

Yes, I've now realisd that it's not ethnicity but cM that are important. I put the number of shared cMs into a free research database recommended by AI and it came back zero for sibling, but in the half sibling range.

OP do either of you have relatives in your matches you don’t recognise?

I recognise all my close matches as they are only four and on my mother's side. But you've made me realise that I don't know my youngest sister's matches. We've never discussed matches.

Thank you. I didn't know that this board existed!

Ok, so if all matches are on your mothers side it’s possible that nobody on your fathers side has tested. You can look at less close relatives, I had to look at people that I shared only about 100cm with to work out who my grandad was.

looking at your sisters relatives would be interesting. If you don’t share the same close 4 matches on your mother’s side then your sister has a different mother. Ime though it’s much more common in this situation that you have the same mother but different dads.

I don't know, as our mum was multi-ethnic. All know is that I'm 50% dad's dna and ethnicitiy and she is 28% ethnicity for the same region and only 26% shared dna with me.

If it was Ancestry, 48% shared dna with your daughter will be accurate as the other 2% is in how the info is recorded. (or something!)

It will say whether the connection is parent 1 = dad
or parent 2 = mum

( and yes. Bloody sexist as ever )

I didn't know this but checked and there's a £1 offer for three months to access the tools to do this. I'll certainly look into it. Thank you!

You can see parent 1 and 2 connections immediately can’t you ?

( I am paid up though so maybe I have more info ??)

That's chimerism. The mother was formed by two embryos merging in utero and the ovaries being from one embryo but the face from the other. So her cheek swap contains different DNA from the ova that became her kids.

Yes you can. It doesn't tell you which parent parent 1 and parent 2 are - they can be either mother or father, so you have to work this out.

I don't think you need to do a membership to see your matches. You need the membership to see family trees.

Its possible you wouldn't need a membership to work out whether you have the same matches or not as if you have enough close matches it will be fairly obvious.

Oh wow really! Thank you for the intel that’s absolutely fascinating. So mum was actually kind of like a conjoined twin. But not a twin. Just one person with two joined functioning and normal half bodies?

The human body is INSANE!

If you get the PRO version you can see shared matches so would easily be able to figure out if paternal or maternal matches weren't on your sisters without involving her.

@ancestrydnashock

OP, in the circumstances and because the outcome will be really important, I would ask King's how much it would be for you and your sister to be tested by them.

They are an accredited lab.

https://www.kcl.ac.uk/forensics/accredited-services/dna-analysis-kings

Oh
ok
thats interesting about the parent 1 and 2 labelling.
I suppose for me it was obvious with other family members also on Ancestry

Thanks !

@TheSquareMile Thank you for the dna lab info.

There is an old saying applicable in this case: Let sleeping dogs lie.
In other words, do nothing and forget all about it.

Not necessarily exactly half and half, but otherwise, you are correct.

"Mine had my daughter sharing only 48% DNA with me (??)"

That sounds entirely right. 50% is the expected amount of DNA shared but that doesn't always happen. There is a website called 'The Shared cM Project' that takes data from 60,000 known relationships.

For a parent/child relationship, the average was 3487 cM (50%) but the maximum and minimum figures were quite a wide spread, going from 2376 cM (34%) to 3720 cM (54%).

My father shares 2491 cM (36%) with his sister. The average for siblings is 2613 cM (38%) but ranges from 1613 cM (23%) up to 3488 cM (50%).

https://dnapainter.com/tools/sharedcmv4

Just to add to the reply from @RedToothBrush , it is a simple matter to look at shared matches.

Go to your DNA Matches page and then click on your sister.

This will take you to the page showing your connection to your sister. There are three tabs, 'Trees' 'Origins' and 'Shared Matches'.

If you click on the 'Shared Matches' tab then that will show all the other people who have done Ancestry DNA tests that share DNA with you and also at least 20 cM with your sister.

So, these are people who are related to both of you.

You then need to work out which side of your family these people come from. If they come from both sides of your family (mother's and father's) then it is most likely that you are full sisters (unless the two different father's were related in some way - eg cousins)**.

If all of these people are just connected to your mother's side of the family then it is most likely that you are half sisters (unless there is an issue with very few people related to your father having done an Ancestry DNA test and so they just aren't on the Ancestry site).

.

** Just as an aside, I did have something similar to this. There was a case in my family going back a bit where there were two cousins with the same name (eg both called William X) who both lived in the same village and both married a woman by the name of Mary within a couple of years of each other.

It really isn't possible to tell which William married which Mary as I have DNA matches from the children of both couples.