Similarity of Siblings

I knew without looking at our DNA that my brother and I were different. He was a blue eyed blond and I was a brown eyed redhead. He is inspired by history and politics while hard core science is what makes me tick. And deeper than all of that, we see the world differently. Still, I had to look.

I inherited 50% of my DNA from each parent, as did my brother. But, the 50% that each of us inherited was not the same. Since I had DNA testing results from at least one parent, I was able to use a utility called phasing at to separate maternal and paternal DNA in both my file and my brother’s file. Then, again in GEDmatch, using those phased files, I did a one to one comparison of the DNA that my brother got from each of our parents to the DNA that I got from each of our parents. I copy-pasted the results into an Excel spreadsheet and then made a spiffy chart. See the chart below!

What I find particularly interesting is that my brother and I have nothing in common on chromosome 23 (the sex chromosomes). Of course we each got a different contribution from our father. My brother got a Y, and I got a presumably un-recombined X. But, I would expect at least some small amount of overlap in the X-chromosome that we each got from our mother and there is not. Either we each got a different whole un-recombined X from our mother, or recombinations happened in a way that yielded entirely unique X-chromosomes for both of us. What are the odds? (I’m sure that someone has calculated this!)

Added Note: It is so unlikely that my brother and I  have zero match on the X-chromosome that we got from our mother that further exploration is necessary. Ann Turner suggested on the DNA-Newbie list that the reason may be in how the software tools handle the sex chromosomes, which is slightly different than how it handles the autosomes. I’ll update the the post after a check, check double-check.

**Ann Turner was correct. The sex chromosomes do not appear to be included in the phased files. It is still possible to compare the phased sex chromosomes, but it will require a slightly different approach that what I was able to use for the autosomes. I’ll redo the comparison of chromosome pair 23 and upload an updated chart and outline of how it was done soon. The discussions of nonstandard inheritance patterns that my error set in motion have been very thought provoking though.




July 17, 2013 · 2:02 am

2 responses to “Similarity of Siblings

  1. Jackson

    Nice blog.

  2. Pauline

    Very good explanation, thanks x