Friday, March 31, 2017

Why don't I have as many DNA matches as ....?

It seems that almost daily someone writes on one of the Facebook genealogy sites, "Why don't I match the person that my relative does?" We have the stock answers that include autosomal DNA is inherited in a random manner. Rarely have I seen this so vividly illustrated as in the results of two of my grandsons. In earlier posts the last several days, I have pointed out that each of them inherited different amounts as well as different segments from each of their four grandparents. If this much variation can occur in just two generations, it can really be skewed as it is compounded over four or five generation.

The first of my three previous posts on this subject demonstrated that grandchildren do not inherit equal amounts of atDNA from each grandparent although it appears that each set of grandparents contribute 50%. To review this post click here.

In a follow-up post I demonstrated that these variances of amounts of atDNA inherited by grandchild can alter ethnic predictions between full siblings. To review this post click here.

In yesterday's post I used Family Finder's chromosome browser tool to illustrate how each segment larger than 5 cM of DNA was inherited by each of my grandsons from each set of their grandparents. To review this post click here.

After my post yesterday, Ann Turner asked if I would be willing to do more analysis on the matching segments the two of my grandsons inherited from their four grandparents by dropping the threshold from 5 cM down to 1 cM. In preparing the data to send to Ann for her analysis, I noticed that the two grandsons had vastly different numbers of matches on Family Finder. One had 23% more than his brother. These numbers were not trivial. One had almost a thousand more than the other. How could this be if they were really full siblings?

It largely depends on who is in the database. Groups from different parts of the world are unevenly represented in various DNA databases. The FTDNA database has a disproportionally heavy presence of Ashkenazi Jews. The maternal grandfather shown below has a high percentage of Ashkenazi ancestry. FTDNA has been able to identify about 90% of his atDNA as being of Ashkenazi origin. His number of matches in the database far exceed the number of matches reported from any of the other three grandparents. Conclusion: the grandson who inherited the most DNA from his paternal grandfather will have the most matches in this database.

Grandson #1 below only inherited 22.1% of his atDNA from his maternal (Ashkenazi) grandfather. Statistically normal would be 25%. He did dip into the treasure trove of Ashkenazi results in the database. 18% of his DNA is shown to be of Ashkenazi origins. However, he did not inherit DNA of that origin to the extent his brother did. This grandson inherited 28% of his DNA from his maternal grandmother whose ancestors came from areas of Central Europe that are underrepresented in this database. She currently has less than 400 matches. So the combination of the DNA he inherited from his grandmother (along with her scant matches) and did not inherit from his grandfather was largely responsible for him having over 900 fewer matches than his brother. 

Matching  Relatives

Grandson #1
Grandson #2

Paternal Grandfather
Paternal Grandmother

Maternal Grandfather

Maternal Grandmother


Grandson #2 inherited 26.2% of his atDNA from his Ashkenazi grandfather. That may not sound like a big difference. However it is approximately a 19% increase from the amount inherited by grandson #1.

Even though the maternal grandfather seems to have won the lottery with more than ten thousand DNA matches, his pedigree chart is so short that we cannot yet begin to sort any of those matches into paternal and maternal ancestors.

The paternal grandmother has an extensive pedigree chart; but as yet she has not added it to her Family Finder account.

The mother (my daughter-in-law) seems to have the number of matches that is near the average of her Ashkenazi father and her match deprived mother.

The father (my son) also seems to have the number of matches that is about the average of his parents.

Why do you have the number of autosomal matches that you do?

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