How autosomal DNA is inherited: 3 siblings compared

If you know how much aDNA you inherited from a grandparent or great-grandparent, can you calculate how much you inherited from that person's spouse?

By now most of us know that each of us get half of our autosomal DNA from our mom and half from our dad. But how we inherit from earlier generations of our ancestors is less well understood. We are generally told after that first generation the inheritance pattern is random. However, we often see charts that show we should expect to inherit about 25% from each grandparent and 12.5% from each great-grandparent, etc. Are there any additional rules?

Below is a small case study that explores these issues:

Inheritance from maternal grandparents are shown in the left hemispheres above and paternal contributions are in the right hemispheres.

The chart above shows how much aDNA each of three brothers inherited from each of their grandparents. Only the middle one, Grandson 2, came close to the 25% average we might expect. However, note that each grandson inherited exactly 50% from his maternal grandparents and 50% from his paternal grandparents. However, the amount inherited from each individual within a set of grandparents varied considerably. The exact percentages can be seen in the table below.

As noted above Grandson 2 came the closest to the theoretical 25% inheritance rate. The aDNA inherited by Grandson 1 was skewed somewhat from that distribution. The practical implications for even this amount of variation can be important to your genealogical research. 

In this example the maternal grandfather is Ashkenazi -- a group that statistically has been over sampled in genetic genealogy databases. He had more than eleven thousand matches in FTDNA's Family Finder. The maternal grandmother came from an area in Eastern Europe very near the origin of her mate; but she was non-Ashkenazi. She had less than five hundred matches in Family Finder. The 4% difference in the amount of Ashkenazi DNA inherited resulted in Grandson 2 having more than five thousand matches while his full brother, Grandson 1, had about a thousand matches less. This also had a noticeable difference in their ethnicity predictions. 

When Grandson 3's Family Finder results are reported, it is expected that his matches and ethnicity predictions will vary ever further from those of his two brothers -- particularly Grandson 2. As we will see below, Grandson 3 inherited more aDNA from his great-grandfather -- the father of his maternal grandmother -- than the 15% he inherited from his Ashkenazi grandfather.

All the results above are based on actual test results. However, if one of the grandparents had not been tested, we could have calculated their contribution. Even if two had not tested we could have calculated their contributions IF we had results from each of their spouses. You have probably already figured out that the contribution of the missing grandparent spouse would be 50% minus the contribution of their known spouse.

[The portion below is being revised to take into account the error in my thinking pointed out by Philip Gammon in his comment below. That revision will be posted tomorrow, October 6th.]

This kind of calculation becomes more relevant in the current case when I took a look at contributions from great-grandparents. We only have actual test results from one great-grandparent who was tested a decade ago about a year before he died in his mid 90s. So we can use his actual test results and also use them to calculate the aDNA contribution of his wife. By now you probably know that the hypothesized contribution will be constructed by subtracting his tested aDNA amount from 25% -- the total contribution of each pair of biological great-grandparents.

For the oldest brother it turns out that the tested great-grandfather had contributed 12.4% of his autosomal DNA. This is very close to the theoretical 12.5%. This left 12.6% for the great-grandmother to have contributed.

 

However, the second and third brothers received a more uneven contribution from this pair of great-grandparents. One received a greater than expected amount from the great-grandmother and the other from the great-grandfather. The tested great-grandfather had passed down 10.3% of the aDNA of the second brother and 16.6% of the aDNA of the third brother. In fact the 16.6% of his overall aDNA that the third brother received through his maternal grandmother and mother was more total aDNA than this brother had inherited from his paternal grandfather (15%, see above). This result was surprising but perhaps not unusual. We are preconditioned to expect that we received about one half the amount of aDNA from each ancestor in each receding ancestral generation. In this case that expectation was confounded because of the large amount of maternally inherited aDNA that the mother of the brothers had passed down to her third son. Although the mother had inherited half of her own aDNA from each of her parents, what she passed down to the third son had disproportionately represented what she had inherited from her mother by a ration of more that two to one -- 35% to 15%.

The second son inherited more of the calculated contribution from his great-grandmother than either of his brothers. He inherited 14.7% versus their 12.6% and 8.4%.

The third son inherited the most aDNA from the tested great-grandfather and did so by a considerable ratio. Although his 16.6% inheritance does not seem like a big difference from the 12.4% of one brother and the 10.3% of the other, it is about 35% more than one and 63% more than the other. In terms of inherited ethnicity and traits, these could matter.

The Takeaways:
   
1. We inherit 50% of our aDNA from each parent; 50% from each set of grandparents; 25% from each set of great-grandparents, etc.
2. We often inherit significantly different amounts of aDNA from each member of a set of ancestors.
3. Even full siblings often inherit significantly different amounts of aDNA from each of their grandparents and great-grandparents.
4. These differences can significantly affect the numbers of autosomal matches reported for each sibling.
5. These differences can significantly affect ethnicity estimates.
6. These differences can significantly affect health and other heritable characteristics.
7. The contributions of some ancestors can begin to fade significantly within two or three generations while those of others can remain robust much longer.
8. If you have test results for at least one grandparent or great-grandparent, you can calculate the aDNA contribution of their partners.

Ancestry.com has done it again! READ COMMENTS!

Ancestry.com has once again made a corporate business decision that emphasizes the ".com" at the expense of the "Ancestry" in the corporate title. The company announced in its official blog today that it will be limiting us to registering only one DNA kit to each membership account except in the case of minor children. The post was entitled "Enhancing Collaboration and Roles on DNA Results". Please read this before you order any DNA kits from AncestryDNA. Ancestry explains this change: 

As the AncestryDNA service has grown, we are increasingly hearing about families taking DNA tests together. Family members want easier and more powerful ways to collaborate with each other to make discoveries in their family story, all while still maintaining control and privacy of their own information. 

A couple of years ago I blogged my thoughts about the appearance that Ancestry believed they always knew more about what we needed as genealogists than we knew ourselves. That post was entitled Does Ancestry think we are NOT OK? and can be read by clicking on this link.

Today's announcement indicates that Ancestry is at it again. The more than two dozen responses posted in the first hour were uniformly those of disappointed genealogists who still are looking for chromosome browsers. They were not looking for Ancestry to force everyone to have a separate Ancestry membership for each kit tested. Peggy Dalton Druck raised a number of other similar decisions over the years that also have seemed counter to the interests of genetic genealogists but perhaps Ancestry hopes they will be favorable to the bottom line of the company. She was kind enough to allow me to include her comments below: 

I was going to order 3 more tests to have done in order to locate relationships within my family. I have been on Ancestry since 1997 and then full time since 2000.Every time you change something on ancestry it deletes a great deal off added things us researchers have added to the pages.I have one main tree and several other trees I research on.Now with this, it is ridiculous.I have stayed with ancestry all these years to prevent my research from being all over the internet.You sold off FTMaker and I can no longer use it for anything because of the last update they did to it messed my ancestry tree up so bad. You got rid of my fathers DNA from his Y 37 test and he is dead now and I can not use his dna numbers anywhere.You were not suppose to destroy dna in case someone wanted to to an upgrade like I wanted to do with my dad’s dna, but now his dead and you destroyed his DNA.Now you are doing this to all of us who have stuck by your company, helped to build the trees we have over all these years and now with our continuing work of verification with DNA you want to stripe us of all of our hard work we have done all these years.

It appears that we will have to deal with even more "orphaned" accounts in the future as a result of Ancestry's implement of this requirement to pay for separate accounts in order to remain actively involved with one's DNA and the matches that are made. As several others commented on the Ancestry blog, many of those we want to test have no interest in or ability to setting up and maintaining separate accounts but would rather delegate this responsibility to the family genealogist. These are often the older family members whose autosomal DNA is most critical for us to collect and preserve.

Ancestry, I wish I had hope that you would listen to us this time.