Skip to 0 minutes and 7 secondsIn this lecture, we will look more closely at how STR test results are displayed and interpreted, the value of SNPs for genealogy, and some other tests that can be taken. Here is a table of STR results for six individuals. This shows the first 12 markers tested. And as you can see, the first five results are a close match. The third result has two differences from the others in the first five-- with the number of matching markers being 10 of 12. This is described as having a genetic difference of two. The sixth result is very different, so the conclusion is that the first five all share a common ancestor while the sixth does not.
Skip to 0 minutes and 59 secondsIn fact, all six have the same surname. There could be several reasons why men with the same surname do not have closely-matching results. Most surnames are multiple origin surnames-- that is, all those with the same surname are not related. There may be a number of unrelated genetic groups-- each bearing the same surname-- whose DNA does not match. Other causes of non-matching are that there has been a change of surname due to illegitimacy, adoption, for the purpose of inheritance, or for some other reason, or simply that there is a lack of test results for men of the surname concerned. I have mentioned genetic distance.
Skip to 1 minute and 49 secondsGenetic distance, which is revealed when comparing results, is an indication of how closely related the individuals are, but no precise predictions can be made because a number of variable factors are involved. Mutation rates-- or the frequency with which mutations in the number of repeats occur-- can vary considerably between families. Over a period of 500 years, perhaps one mutation has occurred while in another family there may have been three. Another important variable is the length of a generation. This is the number of years between the birth of a father and his son in the line being studied and could range from 20 to 40 years.
Skip to 2 minutes and 41 secondsOver a large number of generations, the average length of a generation does average out but can still vary between families. Even a fairly small difference can significantly affect a calculation based on genetic difference. Because of these variables, any calculation of the time to the most recent shared common ancestor is given as a percentage probability-- as you can see in this table.
Skip to 3 minutes and 14 secondsI mentioned earlier that there were two major types of mutation, which are relevant for genealogists. The first involve changes in STRs, which we have already looked at. The second involves mutations known as a "SNP," which is short for "single nucleotide polymorphism." SNPs occur when a single chemical base in the DNA changes at a specific location. In the slide, you can see that the base has changed from an A, which is carried by Males 2, 3, and 4, to a T carried by Male 1. Once this mutation has taken place, it is usually permanent-- unlike STRs, which are prone to further changes.
Skip to 4 minutes and 3 secondsBecause of the stable nature of SNPs, they can be used to identify a specific branch of descendants and have become much more important to genealogists as technological advances have enabled the discovery of many more of these. Up until a few years ago, SNPs could only be used to distinguish between different branches of very large genetic groupings. One of the commonest groupings in Western Europe is known as "R1b," which originated several thousand years ago. SNPs were found, which indicated subdivisions, of R1b-- such as M269, which is itself estimated to date back 4,000 to 8,000 years. This type of information was not particularly useful to genealogists.
Skip to 4 minutes and 58 secondsNow, using advanced testing techniques, it is possible to discover SNPs which occurred hundreds of years ago rather than thousands of years ago. This brings us into the era of recorded history and can introduce an element of precision into test results. Whereas previously, using STR testing, the time to the most recent common ancestor could only be calculated as a percentage probability, SNPs can and have been pinpointed as occurring in a known historical figure. The SNP CLD51 indicates descent from the Macdonalds of Kinlochmoidart, who are descended from John Macdonald, the fifth son of Allan Macdonald, 9th of Clanranald, who died in 1593.
Skip to 5 minutes and 53 secondsAn example of the value of SNP testing can be seen in one of the outcomes of the Battle of Bannockburn family history project run by the University of Strathclyde's Genealogical Studies Postgraduate programme. Lee Macdonald, who lives in Canada, was found to be a close genetic match to the current Macdonald chief of Clanranald using STR testing. Because it was already known from documentary evidence that the chief was a descendant of Angus Og of the Isles, who fought in the Battle of Bannockburn in 1314, the closeness of Lee Macdonald's match confirmed that he was also descended from Angus Og.
Skip to 6 minutes and 39 secondsSince research by Clan Donald had already discovered a number of SNPs, it was also possible to determine that Lee was descended from the Kinlochmoidart branch. This was established as Lee carried the CLD51 SNP.
Skip to 6 minutes and 57 secondsOther DNA tests. Although the Y-DNA test is the most often used, there are two other major types of tests which are available. These are the autosomal DNA test and the mitochondrial DNA test. The autosomal test gives information on all ancestral lines, not just the male line. So this covers your father's mother's line, your mother's father's line, your mother's mother's line, and so on. At present, this test seeks to report results back around five generations with confidence. Matches are usually reported in ranges, such as second to third cousins, fourth to fifth cousins, greater than fifth cousins.
Skip to 7 minutes and 48 secondsThese results may help you to investigate unknown cousins and reveal information about branches of your family which you have been unable to trace using documentary sources. If a sufficient number of close relatives take this test, it is even possible to identify which segments of DNA have been inherited from particular ancestral lines.
Skip to 8 minutes and 14 secondsTwo other types of data can be reported in this type of test. Firstly, there is an indication of the percentages of your ancestry which come from various geographical origins. So, for example, you might be 75% European, 20% Middle Eastern, and 5% East Asian. Finally, information is also given on the X chromosome. So far, this component of the results has not been widely exploited but may become more significant as the body of test results and our understanding of these increases. Mitochondrial DNA testing is the opposite of Y-DNA testing in that while Y-DNA is passed down from father to son, mitochondrial DNA is passed down from mother to daughter.
Skip to 9 minutes and 11 secondsThere is a slight difference, which is that mitochondrial DNA is also passed from mother to son but cannot be passed on by sons. This chart shows how mitochondrial DNA is inherited. Blue indicates those carrying the mitochondrial DNA, but only the females-- indicated by the blue circles-- can pass this on. Another difference-- and a very important one from the point of view of genealogists-- is that there are fewer recent mutations reported in mitochondrial DNA than with Y-DNA. This means that even an exact match between two individuals may only show that they probably shared a common female line ancestor about 400 or 500 years ago.
Skip to 10 minutes and 5 secondsFor this reason, mitochondrial DNA is good for telling us which part of the world a female line ancestor lived in several thousand years ago but not so good at telling us if we are related to an individual with a close genetic match-- and if so, when the common ancestor lived. Here is a summary of the tests I have been describing. A final point worth making-- since it can concern those thinking of taking a test-- is that, in general, DNA testing for genealogy does not focus on health-related information.
In this video we take a look at aspects of Y-DNA testing, its practical applications for genealogists and explore other types of DNA tests that are relevant to genealogy.
Slides from the video are available in the ‘Downloads’ section below.
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