Why is DNA useful when searching for relatives?
DNA is a very new tool in genealogical research. Previously family trees could be assembled through a combination of personal experience and family knowledge; documents and records. Even with a plethora of documents, gaps in a family tree can be left unfilled. In research terms these are called “brick walls”.
DNA testing for genealogical purposes has become increasingly popular in the US over the past few years. At the beginning of 2017 Ancestry DNA announced that it had broken the 4 million mark of testers in its database. Many people initially test because of interest in and “ethnicity estimate” – a breakdown of where your ancestors may have originated hundreds of years ago. Ethnicity estimates are still being refined and are not always accurate. On the other hand, a positive match to someone who is a DNA relative within a database such as Ancestry.com or 23andMe.com can benefit genealogical research in the following ways:
- verify existing research
- establish that two surname variants are related
- provide locations for further genealogical research
- help determine the ancestral homeland
- discover living relatives
- confirm or deny suspected connections between families
- prove or disprove theories regarding ancestry
(List courtesy of the International Society of Genetic Genealogy, https://isogg.org/wiki/Genetic_genealogy).
The 23andMe test
As a genealogist using DNA testing as a tool, it helps to have testers in as many databases as possible. My husband’s DNA is currently in 3 major databases. Each company has a different name for DNA matches. At 23andMe they are called “DNA relatives”.
So what are DNA relatives?
Testing companies basically do the science part for you. In simple terms, here is how 23andMe explains the comparison of DNA for finding relatives:
“When two people share identical segments of DNA, this indicates that they share a recent common ancestor. The length and number of these identical segments is used to predict the relationship between relatives.”
The term “Most recent common ancestor(s)” is used a lot in genetic genealogy and is often abbreviated to MRCA. The types of relationship that we share with most people who are our DNA relatives (other than close family) will be cousinship of some degree. There are many charts available to explain cousinship and the MRCA. Put simply, it works like this:
|If you are……||Then your MRCA(s) are…|
There are other variables to consider, such as half cousins, cousins “removed” once, twice or three times, but I have stuck to the simplest form for now. The testing company compares your “autosomal” DNA (taken from markers along the first 22 chromosomes) to that of everyone else in their database. Areas where your DNA is the same as another person’s are classified as matches. Autosomal DNA matches can be as close as parent/child or as distant as 6th, 7th or 8th cousin. The “size” of the match given indicates the closeness of the relationship. The larger the match, the closer the relationship.
So a few months ago on 23andMe, we got a match with a predicted relationship of third cousin to my husband’s kit. The match had overlapping shared DNA segments on chromosomes 11, 17 and 18. The largest segment was on chromosome 18, an overlap of 66cMs (CentiMorgans, a unit to measure genetic linkage). Totalled with the other segments, the resulting shared match was 109cMs which is in the third cousin range. Cousin L, as we shall call her, was also a match for my brother-in-law and his three teenage children, who had also been tested on 23andMe.
(23andMe Stevensons/”Cousin L”) Chromosome 18 from husband’s DNA “chromosome browser” at 23andMe. Very pretty, but what is it actually telling us?
“Cousin L” had a list of ancestral birthplaces that included Barrhill. This was already interesting. A potential match of a third cousin would mean that husband “D” and “Cousin L” shared a great-great grandparent, exactly what we would be looking for if Cousin L was a descendant of a sibling of D’s great-grandfather.
23andMe then gives you the facility to contact DNA matches and ask them if they would like to explore how you may be related. Many people don’t respond – part of the issue being that a lot of the early testers at 23andMe were only in it for the health and traits report, and were/are still not interested in DNA relative matching. That is slowly changing. Luckily cousin L was very open and chatty about the potential connection. Here is the extract from her reply that gave us hope:
- “I know we are related to Stevensons in Barrhill. My great grandmother in Barrhill was part of that family. There were 4 brothers too.”
Over a series of emails, Cousin L told us the story of her Stevenson ancestors in Barrhill. Could DNA tell us which brother was our family’s direct ancestor? Which one of these four was the great-grandfather of these two cowboys from the 1970s?
Who’s your great-granddaddy? John, William, James or David?
(The top photograph from around 1971 was taken around 50 miles away from the family portrait that the “mugshots” are lifted from. The Stevenson family portrait was probably taken around 1905, judging by the ages of the children.)