Find More Ancestors with Autosomal DNA by Increasing Coverage

When using DNA test results to find evidence for a research subject’s parent, the typical method is to find DNA matches who are descendants of the hypothesized parent. You will only be able to find these matches if you actually inherited DNA from that ancestor. The more distant the ancestor, the less likely that you inherited very much DNA from that person. You are expected to  inherit about:

• 1/4 of a grandparent’s DNA (25%)
• 1/8 of a great-grandparent’s DNA (12.5%)
• 1/16 of a 2nd-great-grandparent’s DNA (6.25%)
• 1/32 of a 3rd-great grandparent’s DNA (3.125%)
• 1/64 of a 4th-great-grandparent’s DNA (1.5626%)
• 1/128 of a 5th-great-grandparent’s DNA (0.78125%)

 

Due to recombination, the amount you actually inherited from each grandparent and each ancestor is not exact. When I compared my DNA test results with my grandparents, I found that I inherited 22% from my paternal grandfather and 28% from my paternal grandmother, not an even 25% from each.¹

Some ancestors aren’t represented at all in your own genome. That’s why your genetic family tree has gaps in it, as Blaine Bettinger has written about. The fact is, you didn’t inherit DNA from each of your distant ancestors.²

This is where the concept of autosomal DNA coverage becomes important. Paul Woodbury defined coverage as follows:

“Coverage refers to the amount of a research subject’s DNA that is represented in a DNA database through their combined tested descendants.” ³

Paul has written and lectured about the importance of testing multiple descendants of your research subject. In his webinar, “Crème de la crème: Targeted Autosomal DNA Testing to Isolate Pertinent Genetic Cousins,” he discusses the concept that different descendants help illuminate different parts of your research subject’s genetic profile.⁴

Example: The Arnolds

In episode 204 of Research Like a Pro, “RLP 204: Recruiting Test Takers for an Autosomal DNA Project,” I discussed a research project I’ve been working on to discover the parents of Daniel Arnold. When I first started working on the project, I had the test results of just one person, a second-great-grandchild. The coverage of Daniel Arnold was only 6.25% at this point, as you can see in figure 1.

Figure 1

Coverage of a parent is estimated by taking half of their child’s coverage. In figure 1, you can see that the test taker’s coverage was 100%, but his father’s coverage was 50%. This is because the test taker only inherited 50% of his father’s DNA. Keith’s father, Elbert, has a coverage estimate of 25%. This is because the test taker inherited about 25% of Elbert’s DNA. The test taker inherited about 12.5% of Amos George Arnold’s DNA. Estimating coverage for one test taker is straightforward – just divide the coverage by 2 each time you go back a generation.

What happens when you add another test taker? The calculation to estimate coverage becomes a little trickier, because each descendant inherited some of the same DNA and some different DNA from the research subject. Paul Woodbury uses a Venn diagram to illustrate this concept.⁵

After reviewing Keith’s son’s matches at Ancestry, I found that his uncle, Elbert’s son, was also in the database, and the manager of his kit shared his match list with me. To illustrate how Keith and his brother inherited some shared DNA from their father, I made a Venn diagram. The middle section shows that they inherited about 25% of the same DNA from their father. See figure 2.

Figure 2

Keith and his brother have some unique DNA from their father, Elbert, and some DNA in common. This helps us see that we can’t simply add up the percentage of coverage of each son when we calculate Elbert’s coverage. We have to estimate how much they share in common and subtract that from the total coverage. Figure 3 shows that we would estimate all the DNA that one son inherited from the father, and then add on only the unique DNA that the other son inherited from the father, without adding in again the DNA that the second son shares with his brother.

Figure 3

Paul Woodbury has given us the calculation for this, where a is half the coverage of child A, and b is half the coverage of child B:

Coverage = a + b – ab

So, to calculate Elbert’s coverage, we take Elbert’s son’s coverage, 100% or 1, divide it by 2, which is 0.5, and plug that in to the “a.” Then we take Keith’s coverage, 50% or 0.5, divide that by 2, which is 0.25, and plug that in for the “b.” Multiplying half of Keith’s coverage with half of Elbert’s son’s coverage gives us the amount they probably share in common from their father which is subtracted out.

a + b – (ab)

0.5+ 0.25 – (0.5 x 0.25)

0.5 + 0.25 – 0.125 = 0.625

As you can see in figure 4, adding Elbert’s son as a test taker increased Elbert’s coverage to 62.5%. It’s so helpful to have test takers who are one generation closer! Dividing 62.5% in half gives us a coverage of 31.25% for Amos George Arnold, and 15.625% for Daniel Arnold. This is more than double the coverage we had before!

Figure 4

After the first two phases of research, we decided to increase the coverage of Daniel Arnold by asking more descendants to share their test results. Three more test takers agreed to share their AncestryDNA results. This increased the coverage of Daniel Arnold from 15% to 31%, shown in figure 5. To figure out the coverage of Amos, first the coverage of his three children were calculated, then the coverage formula for 3 children was used. See Paul’s coverage article for the specific formulas for 3, 4, and 5 children.⁶

Figure 5

As you can see, all the test takers were still descendants of Amos George Arnold. His coverage was increased to 62.89%, but we still have to divide that by 2 for Daniel Arnold’s coverage, bringing the total coverage to 31.45%. Even if we get 100% coverage of Amos George Arnold by all his many descendants sharing their results with me, we still have to divide that by 2 to get Daniel’s coverage, making the maximum coverage possible for Daniel 50% unless we can include test takers from other children of Daniel Arnold.

When trying to decide who should take a DNA test to increase coverage, Paul Woodbury previously recommended testing those who would increase the coverage of the research subject by more than 5%.⁷ However in recent classes, he has taught that even an increase of 1-2% coverage could be worthwhile. In the Arnold case, one more test taker was added after figure 4 was made to show the current coverage. The test taker was another child of Terrell Lorin Arnold, a sibling to test taker “Amos 5.” Do you think this increased Daniel Arnold’s coverage significantly? See figure 5 below.

Figure 5

As you can see, adding test taker “Amos 6” only increased the coverage by 0.48%. While every little bit helps, it would be best to spend targeted testing money on a descendant of one of the other children of Daniel Arnold. Maybe in a future research phase! To read more about the Daniel Arnold research project, including reports from the first phases, go here: https://familylocket.com/parents-of-daniel-arnold/

Conclusion

The more coverage you have, the more relevant matches you will find. This is the exciting part of increasing coverage – discovering matches who help you form or test a hypothesis for the parents of your research subject. In my next post about coverage, I will discuss how matches to different descendants of Barsheba Tharp helped solve the case of who her mother was.

I hope this has helped you think about coverage as you use DNA evidence to help you solve research questions about your ancestors. Best of luck!

Read the next post here: Find More Ancestors with Autosomal DNA by Increasing Coverage Part 2: Barsheba Tharp’s Mother

1. Nicole Dyer, “The Grandchild Report at Gene Heritage,” 26 January 2019, Family Locket (https://familylocket.com/the-grandchild-report-at-gene-heritage/ : accessed 2 June 2022).
2. Blaine Bettinger, ” Q&A: Everyone Has Two Family Trees – A Genealogical Tree and a Genetic Tree,” 10 November 2009, The Genetic Genealogist (https://thegeneticgenealogist.com/2009/11/10/qa-everyone-has-two-family-trees-a-genealogical-tree-and-a-genetic-tree/).
3. Paul Woodbury, “Covering Your Bases: Introduction to Autosomal DNA Coverage'” LegacyTree Genealogists (https://www.legacytree.com/blog/introduction-autosomal-dna-coverage).
4. Paul Woodbury, “Crème de la crème: Targeted Autosomal DNA Testing to Isolate Pertinent Genetic Cousins,” webinar, 6 May 2020, Legacy Family Tree Webinars (https://familytreewebinars.com/webinar/creme-de-la-creme-targeted-autosomal-dna-testing-to-isolate-pertinent-genetic-cousins.
5. Paul Woodbury, “Covering Your Bases: Introduction to Autosomal DNA Coverage'” LegacyTree Genealogists (https://www.legacytree.com/blog/introduction-autosomal-dna-coverage).
6. Paul Woodbury, “Covering Your Bases: Introduction to Autosomal DNA Coverage'” LegacyTree Genealogists (https://www.legacytree.com/blog/introduction-autosomal-dna-coverage).
7. Paul Woodbury, “Covering Your Bases: Introduction to Autosomal DNA Coverage'” LegacyTree Genealogists (https://www.legacytree.com/blog/introduction-autosomal-dna-coverage).