23andMe Ancestry Composition: Comparing Full Siblings

Recently the results of the 23andMe test of my 2nd daughter, Katelyn, became available.

I thought it might be fun to look at the differences in known full siblings Ancestry Compositions.  One might think they should be identical if Ancestry Composition is working correctly, but this isn't so.  Full siblings have exactly the same ancestors but not exactly the same *genetic ancestors*. Not all of a persons ancestors are their *genetic ancestors*.  A person gets 50% of their DNA from each parent, so a persons parents are always genetic ancestors. But because of recombination, a person does not get exactly 25% from each of their grandparents.  In fact, I match my paternal grandmother over 34% and would therefore match my paternal grandfather under 16%. 

Ancestry Composition from 23andMe for Katelyn

She is listed as 99.8 European, broken down to 78.2 British and Irish, 5.2 French and German, 2.0 Scandinavian, 13.3 Broadly Northern European, .9 Eastern European (segment on chromosome 15), and .3% Broadly European, with .2% North African (little spot on chromosome 3).

Next, lets look at my other daughter Samantha. Samantha and Katelyn are full siblings.

Ancestry Composition for Samantha

Samantha is listed as 99.8% European, broken down into 77% British and Irish, 8.7% French and German, .6% Scandinavian, 12.1 Broadly European, .2% Sardinian <.1% Broadly Southern European, 1.3 Broadly European.  And she has the same .2% North African on chromosome 3.

Here is my own Ancestry Composition, as of August 11, 2014. 

23andMe Ancestry Composition for Kristina

As you can see, that little .2% North African segment on chromosome 3 on Samantha and Katelyn is from me. In fact, looking at other relatives Ancestry Compositions shows me that I got it from my maternal grandfather.  I have absolutely no clue where my grandfather got this segment from.