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I’ve recently been matched to two second cousins by DNA. However one shows a 4% match and the other one shows an 8% match. One is on my father’s side and one is on my mother’s side.

Why, or how is one 4% and one 8%?

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DNA inheritance is very random in terms of how much DNA you get from each ancestor, due to genetic recombination. This basically means that although you get close to exactly 50% of your autosomal DNA from each parent, the amount you inherit from each grandparent, great grandparent, etc. will be different.

I was able to test three of my grandparents and I share:

  • 30% with paternal grandmother
  • 20% with paternal grandfather
  • 29% with maternal grandmother

Therefore I have inherited significantly more of my DNA from my grandmothers than grandfathers.

When you extend this to looking at cousins, you will find that some cousins will share more DNA than others, even for the same relationship, because they inherited different amounts (and different segments) of DNA from their grandparents, great grandparents, etc. That is why when looking at resources such as the Shared cM Project, you will always see a range of amount of shared DNA.

You do also have to consider whether you share any other more distant relationships with the second cousin who you share more DNA with. For instance, you could also be a fourth cousin with that person due to other overlapping ancestry, and this would inflate the amount of DNA you share.

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