DNA and Deep Family Mysteries

Many of us have gaps in our family trees. For instance, I know almost nothing of my maternal great-grandmother. Her given names were Emily Maud Alice, she was from Wales, and she married into the Griffin family of Stramshall in Staffordshire, England. But I don’t know her maiden name or anything else about her Welsh family. I’m curious to know more!

For some, these deeper family mysteries take on an additional dimension that goes far beyond curiosity — knowing who that great-great-grandfather was would fill in a critical gap in their understanding of who they are. Many African Americans find themselves in such a position, where through DNA, or other means, they understand themselves to have some ancestry tracing to Europe. Yet that entire fraction of their ancestry remains unnamed, unidentified, unacknowledged.

Shared DNA

A parent and child share 50% of their DNA identically; a grandparent and grandchild share about 25% of their DNA identically. DNA from grandparents is inherited as interspersed segments.

Several guests of the PBS series Finding Your Roots are in this position, as is the host and producer, Professor Henry Louis Gates. Prof. Gates is currently seeking information about his great-great-grandfather along his paternal line — the man who, with his great-great-grandmother Jane Gates, fathered five children, including Gates’ great-grandfather, Edward Gates. Jane Gates was a slave; historical trends suggest that the most likely person to have fathered her children would have been a member of her owner’s family. Gates’ DNA suggests that this man was of European origin — likely from Ireland or Scotland. But Gates would love to know more.

Census records and gravestones can sometimes provide hints regarding the unknown individual. But there is another key source of information that became available only recently with the development of two innovations: genome-wide genetic testing and methods to locate identical DNA segments that indicate that two people are closely related. The first innovation, genome wide genetic testing, became available directly to consumers in 2007. That is when people could order testing of over 500,000 DNA positions scattered across the genome. The second innovation, methods to locate the identical DNA segments that relatives share, became commercially available just over two years ago, at the end of 2009.

With these two innovations, one can now discover previously unknown relatives through DNA. And one can check to see if two people are related as siblings, grandparent-grandchild, first cousins, and so on.

So let’s get back to the question of solving family mysteries.

African American Finding Your Roots guest Cory Booker knew little about one of his maternal great-grandfathers. Prof. Gates and his team found information suggesting that this person was a white physician by the name of S. H. Brown. The team then located one of the doctor’s great grandsons, Mike Hislop, who was more than willing to have his DNA tested to check the relationship. Prof. Gates was then able to reveal to Booker and Hislop that they share several long segments of identical DNA, confirming that they are very likely to be half-second cousins, thereby confirming that, indeed, Dr. Brown was Booker’s great grandfather. As is evident in the 2nd segment of Finding Your Roots, Booker was very moved by this discovery.

2nd Cousin DNA Comparison

Blue segments on the chromosomes 4,5,12,16, and 19 indicate identical DNA segments shared by a pair of 2nd cousins. These segments were inherited from their shared common great grandparents.

How does this technology work? One current method is to compare the DNA of any two people, looking for stretches of millions of positions in a row that are identical. Any two of us are identical at most (over 99.5%) positions in the genome, but only closely related people are identical for long stretches of DNA. Every child inherits one copy of her genome from her father and one from her mother. So the child is half-identical across the whole genome with each parent. A child shares about 25% of his DNA identically with each grandparent. More distant relatives share much less identical DNA. First cousins typically share about 12.5%, while 2nd cousins typically share 3%. By chance, however, 2nd cousins may share twice that much, or half as much. The possible relationship need not be along strictly maternal or strictly paternal lines for DNA to show evidence of the connection.

Using this DNA technology, Prof. Gates was able to reveal to another Finding Your Roots guest, Brown University President Ruth Simmons, that she is a biological descendant of the slave-owning Beasley family of Grapeland, Texas. The team located a white descendant of Charles Beasley – Camber Haymen – and compared Camber’s DNA to Ruth’s. They found three shared DNA segments, one of which is about 20 million DNA positions long, revealing that Ruth and Camber are likely to be 4th or 5th cousins.

Another Finding Your Roots guest, Geoffrey Canada, has long been curious about his “Canada” line. His Y chromosome indicates that this line traces back to a white man. Prof. Gates and the producers of Finding Your Roots have been searching for further detail, and have learned through paper records that the name Canada was derived from the name Cannaday. But they have not yet found DNA evidence connecting Canada to any particular Cannaday ancestor.

For people looking for evidence of connection to a possible ancestor, DNA evidence may or may not remain: if that possible ancestor lived many generations ago, then very little of their DNA may remain in any living descendants. And the chance that two descendents on different lines inherited the same segment drops quickly each generation. Third cousins are very likely to have a detectable matching segment of DNA, but 6th cousins are very unlikely to have such a matching segment. More on this.

So what of Prof. Gates’s quest to find out who fathered five children with his great-great-grandmother Jane Gates, including his great-grandfather, Edward Gates? Gates suspected that it was his great-great-grandmother’s slave owner who’d fathered the children, but in the last episode of Finding Your Roots, he was disappointed to learn that DNA showed no evidence of this connection. His quest to solve this deep family mystery continues.

About the Author:
Joanna Mountain is a geneticist who is a consultant to the PBS series “Finding Your Roots,” and consulted previously on the PBS series “Faces of America.” Dr. Mountain completed her PhD in Genetics at Stanford University and has spent over 20 years studying human genetic diversity. Currently, she is Senior Director of Research at 23andMe, Inc.