How genetics can help genealogists
As you saw in the example given in the short video, the Y chromosome was passed down the male line, from John senior, to his three great-grandsons John, David and Hugh.
Although it does occasionally mutate, this only results in small changes, so in our example, the great-grandsons will carry a Y chromosome very similar to their great-grandfather, and, for that matter, to their direct male line ancestor several hundreds of years ago.
The most commonly used type of Y-DNA test is the STR test which aims to find similarities between results, since closely matching results indicate descent from a common male line ancestor. STRs or Short Tandem Repeats are repeated patterns of chemical bases which can be measured in the Y chromosome. A pattern could be repeated perhaps 12 times in sequence at a specific location on the chromosome.
Y-DNA STR tests used for genealogy measure the number of repeats at selected locations, the most popular tests now using 37 or 67 locations. These are described as 37 marker tests or 67 marker tests. When test results are compared, the closer they match one another, the more closely related are the individuals who have tested. Due to mutations that occur in the STRs, patterns at one of the locations could be repeated 11 times instead of 12 times or perhaps 13 times. Now let’s go back to our video example.
A comparison of the 37 marker test results for John, David and Hugh show that John and David match exactly, while Hugh has a difference of one at one of the markers. This level of matching is what would be expected and confirms the documentary evidence that they are closely related, since they are second cousins.
You should note, however, that these test results can only confirm that the relationship is a close one and they are not precise enough to show that the relationship is that of a second cousin. If test results, when compared, show major differences, this indicates that the individuals are either very distantly related or not related. Tables can be consulted which indicate what the level of probability is that they shared a common ancestor a certain number of generations ago.
One possible outcome when matching test results is that the majority of matches have a different surname from that of the individual who has just tested. This may be because the matches are not particularly close and the common ancestor lived before the time when surnames became established.
On the other hand, if the matches are relatively close, the likelihood is that at some point there has been a change of surname which could occur for various reasons, including illegitimacy. The male line descent will almost certainly be that of the surname found most frequently amongst the close matches.
One other point to be aware of is that matching of STR results can be very useful for the period from around 1500, but before that, is much more unreliable. For the early period other tests are more important as we will explore elsewhere.
The resources in the ‘See Also’ section below can be consulted for more information on the topic. These books may be of interest to you as well:
Emily D. Aulicino. (2014) Genetic genealogy: the basics and beyond. Bloomington, Indiana: AuthorHouse LLC.
Debbie Kennett. (2011) DNA and social networking: a guide to genealogy in the twenty-first century. Dublin: The History Press.
George Redmonds, Turi King, and David Hey. (2011) Surnames, DNA, and Family History. Oxford: Oxford University Press.
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