DNA and Genetic Genealogy

Summary

The collective DNA of people alive today contains information about our ancestry. Direct to consumer DNA testing for genealogical purposes started in 2000 and is now mainstream. This section provides an overview of the process and the types of genealogical insights which can lead to. The linked sub-subsections describe in more detail the types of tests available, the science underpinning them, and how the results can be used to provide genealogical conclusions. There is a sub-section focussing specifically on the application of DNA testing to the pre 1700 Isle of Man.

My aim is to provide an objective overview of the field. Genealogical DNA testing is typically carried out through one of a handful of commercial companies. Along with thousands of other people, I run a free genetic online genealogy project hosted through one of these companies. I do not receive any financial or other benefit either from this company nor from any organisation related to genetic genealogy however, and this site does not carry advertising. Please contact me at robert@manxmanorialroll.com to highlight any inadvertent errors or omissions on this site.

Overview of the Process

The typical process is for the genealogist to select and purchase a test online. The relevant company then sends a pack for the tester to return a DNA sample. (This is typically taken from a cheek swab or saliva). Over a period of some weeks (with the duration depending upon the specific test) the provider analyses the returned sample to extract genetic markers. These markers are specific sections of the genome which are chosen because of their likelihood to vary amongst the population. Once the laboratory testing is complete, the user will receive details of the results for each genetic marker. The major genealogical testing companies have large databases of test results, and (subject to user permission) they will also use close matching of genetic markers to highlight and provide contact details of likely relatives.

Although some types of DNA tests can directly reveal information about ancient ancestry, getting best value out of the results relies careful analysis on the combination of genetic and genealogical information from multiple people. The idea is to form a genetic family tree showing the descent of hereditory generic variations through ancestors or ancestral lines. This genetic tree can then be used to make genealogical deductions.

Overview of Genealogical DNA Tests

The most commonly taken test of this type is for Autosomal DNA. If two individuals share any recent ancestor then it is likely that they will both have inherited some of the same autosomal DNA from that common ancestor. If so, then a comparison of their test results will show a partial DNA match. The testing company typically provides a list of all potential matches with other testers, enabling various ways for the testee to expand their family tree. The chance of a detectable correlation reduces as we look for more distant cousins, and typically won't help us go back beyond about 8 generations. Some companies also use the results to provide a rather crude estimate of an individual's ethnic origins.

Y-DNA tests were the first to be used for genealogy, and are still going strong today. They exploit the fact that Y-DNA is passed down the male line with hardly any changes. The Y-DNA test is very well suited to one-name studies and there are individual Y-DNA projects for many thousands of surnames. Unlike autosomal testing, the approach is viable for distant ancestors and indeed the more refined tests allow the testee to be placed on a "tree of mankind" which provide an accurate if sketchy male line ancestry back to the first human. It is by far the most useful test for pre 1700 research, and when combined with conventional genealogy is a powerful tool. As with all DNA testing, its effectiveness depends on the number of other relevant testees. Specific limitations of the Y-DNA testing are that it can only be taken by males, and will only help with one family line. It is also somewhat more expensive than autosomal testing.

Although we all have Mitochondrial DNA, it is inherited only from the mother and the Mitochondrial test is the female line counterpart to Y-DNA testing. Unfortunately the average amount of variation per generation is much less in mitochrondrial DNA than in Y-DNA and this makes it much harder to reliably predict the existence or closeness of a relationship between two testers.

Some Autosomal Tests also incorporate X-DNA testing. The results require some skill to use, but can provide additional help with specific ancestral lines. It potentially enables us to go several more generations for those favoured lines.