Taking a DNA test and analysing the results can be a great help in digging deeper into your family history. You can make new discoveries about your family’s past and it can give hints to guide your research and help you connect with new relatives. It can also help you to confirm information in your tree.
In this blog I’ll provide a whistle stop guide to the three main types of DNA test available and explain what information these provide.
To begin, let’s try to recall your school biology classes. Everyone has 46 chromosomes, arranged in pairs twisted together; the famous ‘double helix’. One of each pair comes from our father, the other from our mother. 22 of these pairs are termed the Autosomals. The remaining pair are the ‘sex’ pair and define whether a person is male (XY) or female (XX). All of these have a part to play in DNA analysis to determine ancestry.
In addition, there is Mitrochondrial DNA. Unlike the chromosome pairs, which are concentrated in the nucleus of our cells, Mitrochondrial DNA is distributed throughout our cells and so is far more plentiful.
Autosomal DNA (atDNA)
This is the DNA type that is most commonly tested in the industry. atDNA maps our 22 Autosome Pairs which contain the DNA of all our recent ancestors. The thing to remember about atDNA, however, is that our DNA can only carry a set amount of information, so with each generation, statistically, the contribution from each of our ancestors is halved. So atDNA testing is only considered good for detecting relationships back around 5 to 7 generations.
The Autosome Pairs are something of a melting pot, where the various elements of our parents' DNA are mixed to create us. This is not done in an ordered fashion, however, so some strands are passed intact to us while others are diminished or even eliminated. This is worth bearing in mind when someone is boasting of being descended from Lord Such-and-such in the 1200's. They might very well be so, but it could also be the case that this ancestor's contribution to their DNA has been so diminished over the years that it has disappeared altogether.
Y-chromosome DNA (Y-DNA)
Y-DNA is passed down the direct male line from father-to-son. The test can only be done on men as women do not have the Y chromosome in their ‘sex’ pair. Although Y-DNA represents only a tiny fraction of our overall lineage, it is important in that it remains largely intact when it is passed down through the generations. This allows ancestors to be traced back Centuries or even Millennia. There have been fascinating studies recently about descent from famous Scottish and Welsh Kings and Warlords. It is the Y-DNA that is being plotted in these.
An exciting thing about Y-DNA is that it mutates at a very slow, but predictable, rate. This allows a pretty good estimate to be made of when 2 individuals shared a common ancestor. So, if we have, say, 100 individuals who are all descended from a common ancestor born 2,000 years ago, a family tree can be drawn up of when each person's branch of the family broke away from the main trunk. We thus get a workable tree long, long before there were written records.
Although women can’t take the Y-DNA test themselves they can still potentially benefit from the data by getting a close male relative, for example a father, uncle, brother etc., to take it on their behalf. A son cannot help as his Y-chromosome will have come from his father so from an entirely separate ancestral line.
Mitochondrial DNA (mtDNA)
mtDNA is passed down the direct female line from mother-daughter. Compared to atDNA or Y-DNA, mtDA exists in very large amounts in our cells. This means that it mutates at a much slower rate than Y-DNA - we are not talking Centuries, we are talking tens of Millenia. There is a DNA initiative at work at the moment which is seeking to plot the movement of modern humans around the world since the earliest times. It is mtDNA which is being used to do this.
In summary, atDNA tests cover your whole family tree but are only helpful in analysing the most recent generations. mtDNA and Y-DNA only trace back through one particular line – a tiny fraction of our overall DNA make-up – but can reveal ancestors going far further back in time.
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