16 May 2012

A Half-Brother's DNA

      During Family Tree DNA's recent DNA Day Sale, I purchased a Family Finder test for my half-brother, Allen. Since I'd already done a y-DNA test for him, he still had a DNA sample on file. Thanks to this, I was able to get his test results back in less than a month.

     You can see the chromosome chart on the left, which shows the DNA I share with my brother. Because we share only one parent, we share less DNA than full siblings: an average of 25% vs 50%.

     Allen and I share 1,610.77 centiMorgans of DNA in 51 segments. What's this mean? Honestly it can be hard to understand. For me, this number has the most meaning when seen in comparison to other results. For example, my mom shares 2377.55 and 2324.45 with her full brothers.

     This is one aspect where I think 23andMe's results are better, as they present these results as a percentage. It can makes DNA results easier to understand for the average customer. Another problem is that this chromosome view does not differentiate the difference between full and half chromosome matches (which again, is something that 23andMe does do).  Thus, the ftDNA chromosome chart of full and half siblings resemble each other in a somewhat misleading way.

     I'm getting a little off topic, but look at the different ways that the DNA comparison between my mom and one of her brother's is represented by the two different companies. On the left, ftDNA shows similarities in orange. On the right, 23andMe shows similarities in light blue and dark blue (half and full  matches, respectively). The match areas are the same, but 23andMe gives more detailed results.

     Other results from my brother's DNA test include his population finder results:

1 comment:

Paul said...

I'm curious to know if you've compared yourself from 23andMe to FTDNA (or AncestryDNA) via GEDmatch. Here's why I ask:

I've tested my two sons at 23. Compared to my 23 kit, they share roughly 3600 cM with me (each).

Taking their results from 23, and comparing to my results from AncestryDNA, I get a slightly lower value (but still very close to 3600 cM each).

Taking my AncestryDNA kit and comparing to 23andMe results (for me in both cases), I get about the same value (3600 cM). This is contrary to what I expected -- I expected nearly 7200 cM (or 7100, or something larger).

Completely agree with you that the % reading at 23andMe is a bit more useful, BUT, their filter level is so high it rejects all the small bits. For example, at 23andMe, I have a verified cousin at 94 cM, but at FTDNA, she comes in at over 125 cM. That extra 31 cM is pretty significant, as it jumped us an entire generation when we were trying to find where we were related.

Thanks for posting.



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