My half sister and I share the same father, but different mothers. Because my paternity was questionable we both did an Ancestry DNA test....I matched to relatives of my grandmother and she didn't....and we didn't match to each other. I suspect we don't share the same father but she insists we do, so we uploaded our DNA to MyHeritage- and again no common matches. our father was 100% Swedish and I come up with hundreds of DNA matches from Sweden.. she comes up with 8, but her heritage comes up as having no Swedish ancestry.

Is there a scientific explanation for these results besides us not sharing a father?

I would think that these tests are accurate enough to find common DNA among half sisters or they would have been shut down due to all the grief and drama that would be caused my misinformation leading people to think their life is a lie.

3 Answers 3


The tests can be trusted. You are not half-sisters.

Paternal half-sisters, minimally speaking, share an entire X chromosome, as females inherit one X chromosome from their father. Beyond that, they share about 25% of their DNA all told.

I recommend reading either of Blaine Bettinger's books on genetic genealogy. Your local library may have copies.


If you are at least half-sib you would still share TONS of DNA; no matter if they tested 23rd chromosome or not. I have a half-sister, and we share a lot and are inside "the range" listed on the 2017 chart done by Blaine Bettinger: https://isogg.org/wiki/Autosomal_DNA_statistics#/media/File:Shared_cM_version_3.jpg

If you both share no DNA -at all-, she cannot be a half-sibling. Maybe your father really thought this was his child. So you see, if you pursue this, you will take away her only father. Sometimes we have to keep mum to protect relationships or someone's self-identity in life. This is the downside to being a genealogist.

  • 1
    Full- and paternal half-siblings of the same sex must share at least one chromosome. However, it is possible for opposite sex full- or maternal half-siblings to share no DNA (ignoring mitochondrial DNA). It's just that the odds of that happening are ridiculously small (1 in 2^46, about 1 in 70 trillion) for full siblings. OTOH, for half-siblings, the odds are 1 in 2^23, or about 1 in 8.4 million, which while very unlikely, is a high enough probability such that it's likely there are many such cases existing in the world.
    – Makyen
    Commented May 14, 2019 at 0:05

That was a lot of family drama for no reason.....neither ancestry or myheritage use the 23rd chromosome in their DNA data---so it is very possible to not come up as a match for a half sibling. Gedmatch.com does analyse the x chromosone though so if anybody having the same experience stumbles on this, upload your raw DNA data there and then use the 'one to may' link and click on the x chromosone ☺️

  • Do you match on your autosomal (non X) DNA?
    – user6485
    Commented Jul 25, 2018 at 14:26
  • No we don't, but I don't think that's unusual for 1/2 siblings....
    – Jbr
    Commented Jul 25, 2018 at 16:50
  • I'm sorry but it would be very unlikely for you to share no DNA -- on average you'd expect 25% -- isogg.org/wiki/File:Shared-cM-Project-Image-2.png as per the earlier answer.
    – user6485
    Commented Jul 25, 2018 at 17:13
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    I'm sorry. You are not half-sisters -- the odds of you sharing no autosomal DNA are so small as to be effectively zero.
    – user6485
    Commented Jul 26, 2018 at 5:32
  • 1
    How much of a match was there on the X chromosome? Commented Jul 29, 2018 at 15:47

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