I recently got DNA results for my dad’s paternal first cousin. She is the daughter of my dad’s father’s brother, making her my first cousin once removed.
The percentages of DNA that she shares with our relatives are all over the map. I found these results fascinating because it shows that recombination is random & incredibly hard to predict.
Leona, my dad’s first cousin, has two first cousins that have also tested their DNA, Dan and Aletha. The theoretical percentage shared by first cousins is 12.5%. Dan and Aletha both deviate from this percentage – Aletha shares more than expected, while Dan shares less. This kind of variation is pretty common and not unexpected.
Things start to get more extreme when I look at the comparisons between Leona and my three sisters & I.
One of my sisters shares more than twice as much DNA with Leona as my other sister.
Leona and my sisters and I are first cousins once removed, and are therefore expected to share only half the amount of DNA as first cousins (6.25%). However, Leona actually shares more DNA with one of my sisters than she does with her true first cousin. This is pretty odd – it just goes to show that recombination is often very hard to predict.
The comparison between Leona and my sister and her two daughters further demonstrates this fact. My sister and Leona are first cousins once removed – this relationship has an expected theoretical percentage of 6.25%. My sister’s children and Leona are first cousins twice removed – this relationship has an expected theoretical percentage of 3.125%.
My sister has a slightly lower than expected percentage of shared DNA with Leona – 5.28% as compared to the expected 6.25%. A mother passes on 50% of her DNA to her children, so it would be fair to assume that the children should share about 2.64% with Leona. This is not what we see, however. By random chance, one of her daughters happened to inherit much more than 50% of the shared DNA, while the other daughter happened to inherit much less.
Even though her mother shares less than expected with Leona, daughter number one inherited a large percentage of Leona's shared DNA from her mother (about 60% rather than 50%). As a result, her percentage with Leona is almost identical to the theoretical percentage for first cousins twice removed – 3.10% compared to the expected 3.125%.
In contrast, daughter number two inherited hardly any of her mother’s shared DNA with Leona. (about 20% rather than 50%). As a result she shares only about one third of the expected percentage – 1.16% compared to the expected 3.125%. 23andMe predicts that they are third to fourth cousins because they share such a small percentage of DNA.
I have really enjoyed looking at our comparisons -- I think it is very interesting the way it can vary so much! Please comment if you have any questions or need any clarification.