Genetic Genealogy: Chapter 5

 Genealogical Applications for atDNA

Exercises for Chapter 5

1. Sisters with an AncestryDNA test get results; one matches to a predicted third to fourth cousin; the other does not. Should they both match? Why or why not.

They could both match, or neither. About 90% of third cousins are detectable DNA matches; only about half of fourth cousins cousins.

2. Fred is testing numerous relatives in an attempt to map his chromosomes. He shares no detectable DNA with Victoria, a seventh cousin, who is a descendant of fifth great-grandparents. Can Fred conclude that these Quincy ancestors are not in his genetic family tree?

No, testing one distant relative with whom he is likely not to share detectable DNA proves nothing. It is simply a lack of evidence.

3. Roy and Mike share a single 6.43 cM segment of DNA. Which company or companies will show them as genetic relatives? Which will not.

At the present time, none of the testing companies will show them as matches. While AncestryDNA reported matches down to 5 cM when the book was written, their matching threshold is now 10 cM. They could both upload to Gedmatch and lower the matching threshold to 6 cM and see their match there.

4. One cM is equivalent to a percent (one in one hundred) probability of a recombination event occuring in that DNA segment within a single generation. Is it possible to share a DNA segment of a 100 cM or greater with a relative other than a parent passing down entire chromosomes to a child?

It is possible to share a segment of over 100 cM with other relatives. My longest shared segment with a first cousin is 117 cM.

5. Viktor has tested his parents but not himself. His grandparents are living but he's not tested them. He has an aunt, a first cousin, two second cousins and a third cousin. Whom should he test first?

His grandparents. Viktor is right about the DNA he got from them through his parents, but he is missing the 50% from each that he did NOT get from his parents, because they didn't have it to pass to him. In addition, once they pass, the chance to test them is gone. The second cousin and then third cousin after that are most important, because they allow pinpointing great, great-grand, and second great-granparents. The aunt and first cousin are the least valuable unless their own children want them to test for their own reasons. 

6. Sally has tested three of her grandparents. Shared DNA with each follows; what percentage of DNA did Sally inherit from her untested grandfather?

Adding the total of all three tested grandparents and then subtracting that total from 100 yields 21.4% which she would have inherited from her untested grandfather.

7. Place an arrow or arrows showing where a recombination event occurs.

8. Draw on the blank chromosome figure to show the segment or segments the granddaughter would share with her paternal grandfather.

not to scale!

The segments in-between the segments in the previous image.

9. Laura has tested herself and her grandmother Brenda. On chromosome 4 they share [diagram] DNA segments. She also tested her son Michael and would like to compare his DNA with that of Brenda, his great-grandmother. Which of the following chromosome browser views should she expect to see?

a, b and c although d is possible *if* her father shared some DNA with her mother's mother. 

Use family tree chart for Q 10-15

10. Violet Redden, the family genealogist, has tested her self as well as five of her family members, bolded in the family tree above. Complete the table determining a) the relationship to Violet and b) expected amount of shared DNA in percentages and cM. 

11. Violet receives Eryn Tisby's results and discovers that Eryn shares 205 cM of DNA with Marcelle Sparks. How does that result align with the family tree? If the result is unexpected, what piece of information from the family tree might explain the result?

It is unexpectedly large for the known relationship. This means it is likely that Eryn and Marcelle share more than one genetic relationship, and more genealogy research is needed.

Violet downloads her raw data and the new data of Eryn Tisby, Elijah Sparks, and Marcelle Sparks. After an upload to Gedmatch, she gets the following results. 

Use table for Q 12-14

12. Determine the most distant ancestor in the family tree to whom the DNA segment that Violet shares with Elijah Starks on chromosome 18 can be mapped tentatively.

Brady Sparks and Minerva Ayers, although it is possible that there are other genetic relationships to be found.

13. Determine the most distant ancestor or ancestors in the family tree to whom the DNA segments that Violet shares with Eryn Tisby on chr. 1, 5 and 12 can be tentatively mapped.

Rene Tisby and Dori Sparks are ancestors in common, but the high cM between Eryn Tisby and Marcelle Sparks suggests other shared ancestors not yet in the tree.

14. Based on the information in the tree and the table, describe what is unusual about the DNA that Marcelle Starks shares with Violet.

They share a segment on chr. 8, but Marcelle's ggrandfather Elijah Starks does not share that segment with Violet. There is no one else on the tree (so far) who could have contributed that segment to both Violet and Marcelle.

15. A chromosome browser reveals the following pattern on chr. 5, with three genetic markers sharing a long segment of DNA (35 cM, 50 cM and 75 cM) with the test taker: [figure]. Based on this view, what do we know about the genetic relationship of the three individuals to one another?

Not very much. We don't even know if the two matches with the test-taker match one another. One could be on the maternal side and the other on the paternal. The segments are reasonably large though so they are presumably matches. The ICW tool might shed some light, and perhaps an email to each about their own matches might shed even more.

Use for Q 16-17. Share DNA: X

16. Place the nine individuals listed in the table into potential triangulation groups based on the segment data and the matrix tool results.

Aaron's match is tiny, and he does not match with any of the others, so I would call it a false match. Brenda, Leonard and Shana form a group for further study, as do Ellen, Susan, Johnny, Georgia and Donna. Brenda is possibly the closest relative so I would start with her.

 

17. What is the limitation of using this approach for triangulation? What additional piece of information would you seek to verify that those potential groups are true triangulation groups?

Without comparing each of members with one another on the chromosome browser or Gedmatch's one-to-one tool, you do not know for sure that they all share the same segment of interest.

18. Using the table below, calculate the completeness of your genealogical family tree back at least seven generations (to sixth cousin relationships), where completeness is the percentage of ancestors for whom you have a name or some identifying information. Consider the reliability or accuracy of that information, especially in the older generations where most distant-cousin relationships are being compared. How might this impact the search for shared ancestry with identified genetic matches? (The "Total # of Possible Ancestors"column may change if there have been recent cousin marriages.)

19. Angeline would like to identify unknown parents of fourth great-grandfather born in the 1750s. ... Can Angeline use atDNA to help identify this couple? If so, how many cousins should she test?

It is possible that she can find many descendants of this couple by doing very complete trees for each of the cousins she finds and comparing their results with one another. And perhaps she can find descendants of the most likely parents of her ancestor and test them, although there she is getting into dealing with very small segments and the high probability of "by chance" segments. This could be an expensive, slow project!

20. David Welch believes his second great-grandfather to be part Native American. There are no records supporting this belief, and atDNA results from the testing company reports no Native DNA. Is David's hypothesis still possible?

Since David only inherited on average 6.25 percent of his ancestor's DNA, unless he was 100% Native American, it would likely not show up in an atDNA test. 

It is possible that finding a paternal line descendant to test via Y-DNA could have some results