Is it possible that I do not share any DNA with my blood-related cousin?

More and more people are getting their DNA tested now. You just send your saliva to companies like https://www.myheritage.com, https://www.familytreedna.com/ or https://www.ancestry.com/. A few weeks later you get your DNA test results and you will be able to find relatives from all over the world on those websites.

I did it and found a lot of interesting relatives, but also some very strange things…

For example, my sister found a close cousin, who didn’t show up in my list of relatives, but someone else from the same family did show up.
First I thought there might have been adultery in play, but actually that must not be the reason at all.

In fact you do only inherit 50% of your mom’s DNA and 50% of your dad’s DNA, when you are a woman. As a man the ratio is 51% mom and 49% dad.
That also means that you only got 50% of 50% of your grandpa’s DNA, which is in total only 25% of his DNA. The same is true for the DNA of your grandma.

This explains why there is a chance that you do not share any DNA with a cousin. In this case you are still blood-related but you did not inherit the same DNA from your ancestors.

To make it easier for you, just have a look at the following simplification:

Your grandpa’s DNA (split in 8 parts)
GP1 / GP2 / GP3 / GP4 / GP5 / GP6 / GP7 / GP8

Your grandma’s DNA
GM1 / GM2 / GM3 / GM4 / GM5 / GM6 / GM7 / GM8

Your mom’s DNA from your grandpa (50%) (for example)
GP1 / GP2 / GP3 / GP4

Your mom’s DNA from your grandma (50%) (for example)
GM1 / GM2 / GM3 / GM4

Your mom’s full DNA
GP1 / GP2 / GP3 / GP4 / GM1 / GM2 / GM3 / GM4

Your DNA from your mom’s line (for example)
GP2 / GP3 / GP4 / GM1

Now we do the same for your cousin!

Your grandpa’s DNA
GP1 / GP2 / GP3 / GP4 / GP5 / GP6 / GP7 / GP8

Your grandma’s DNA
GM1 / GM2 / GM3 / GM4 / GM5 / GM6 / GM7 / GM8

Your aunt’s DNA from your grandpa (50%) (for example)
GP2 / GP3 / GP4 / GP5

Your aunt’s DNA from your grandma (50%) (for example)
GM3 / GM4 / GM5 / GM6

Your aunt’s full DNA (for example)
GP2 / GP3 / GP4 / GP5 / GM3 / GM4 / GM5 / GM6

Your cousin’s DNA (50% from your aunt) from your mom’s line COULD just BE
GP2 / GP3 / GP4 / GP5
While your DNA in this example was
GP2 / GP3 / GP4 / GM1

-> means you would share GP2 / GP3 / GP4 (75%)

OR (for example)

GM3 / GM4 / GM5 / GM6
While your DNA in this example was
GP2 / GP3 / GP4 / GM1

-> which means that you would not share any DNA from your mom’s line with your cousin!

Theoretically it could even happen to siblings that they do not share any DNA, because they do get only half the DNA from mom and dad, and the another sibling could just have gotten the other half, but it’s very unlikely.

In any case… the further you go back in time, the more likely it is that you do not share common DNA with blood-related relatives. If you actually have a sibling, there is a good chance to find “missing” relatives in your sibling’s DNA match list, even when those relatives does not show up in yours. As long as the DNA match list shows your sister as your sister, this also means that you are related to the people your sister is related to, but as mentioned, you just did not inherit the same DNA.

Another astonishing finding: My sister does have around 600 relatives on FamilyTreeDNA, but we do share only 200 of them. More than 400 relatives do not show up in my FamilyTreeDNA match list and vice versa.

Please ask me any questions about that, because I also want to find out more about it.

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