DNA matching: Half siblings

My DNA quests continue.  My mother's half brother was kind enough to submit his DNA specimen to 23andMe.com.  I am busy analyzing the results.  They share 26.6% of their DNA.  Full siblings share about 50%.  To interpret, each of them has about 75% of unshared DNA.  25% is from their mother, and 50% from their respective fathers.  If a genetic distant cousin matches one half sibling and not the other, we still do not know which parent holds the relation.  But if a genetic distant cousin matches both half siblings, then the relation is through their common parent.  Here is what my mother's comparison to her brother looks like.

23andMe.com
Autosomal DNA comparison of half-siblings (sister and brother sharing mother)

My uncle matches me 14.2%.  This is roughly half of the match between him and my mother.  This is because approximately half the DNA is lost from one generation to the next.

23andMe.com
Comparison between my mother, her half brother, and me.

This information is very valuable because anyone who matches both my mother and her brother matches on my maternal grandmother's side of the family.  The dark blue areas in the above graph are areas of my DNA that are identical to my maternal grandmother.  You can see how some segments break into tiny pieces quickly, while other stayed intact through a few generations.  The blank areas contain some more of my maternal grandmother's DNA along with my paternal grandfather's DNA.

The difficulty with identifying the common ancestor of genetic distant cousins is that you have two possible paths to follow at each generation as you travel back.  I know which parent to credit my matches to because both of them are in the database.  But beyond them, I do not know which of their parents share the relation, and so on.  At least now I have a better chance of identifying common ancestors by determining which ones match my maternal grandmother.  I will be explaining more and sharing examples in upcoming posts.