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January 21, 2015 / sennettfamilytree

Genealogical DNA for Dummies

I’m sure someone must have already written a “Genealogical DNA for Dummies” guide, but here is my attempt to explain it in non-technical terms.

Men have an X and a Y (chromosome) that are paired together. Women don’t have the Y, they just have two X’s. A child’s genes come from a mix up and recombining of the two parents. So a girl child will still end up with two X’s but some bits of them will come from the father’s X and some from the mother’s. A boy child on the other hand may have some bits of X from both mother and father, but his Y will have just come purely from his father – virtually unchanged. That makes Y-DNA such an exciting possibility for genealogy where you want to follow the paternal (surname) line. You could expect that Y-DNA will therefore pass virtually unchanged from father to son through the generations, meaning that the Y-DNA of a man’s g-g-g-g-grandfather will look very much like that of his own Y-DNA – with some little changes.

The little changes are where “mutations” occur over generations – these aren’t necessarily bad, it just means that the gene has been copied slightly differently as it passed from father to son. Because its possible to predict how often mutations are likely to occur, comparing the Y-DNA from distant male cousins with a common ancestor (and seeing how many differences there are in a standardised set of markers tested) allows a rough estimate of when that common ancestor might have lived. A very close match between two men who share a common surname (only one or two differences) makes it very likely they are related, and a bigger number of differences makes it either less likely they are related, or that the most recent common ancestor is very many generations back.

So how does this work for genealogy?
Lets suppose, as we have in the Sinnott/Sennett one name study, we have a whole lot of families who all believe their common ancestor came from Co Wexford, but the paper trail for conventional genealogy research has dwindled away, or we suspect that John and James who emigrated to the US about the same time were brothers but there is no documentation to prove it – genealogical DNA studies can now be used to show whether its possible that two families (that are at the moment quite separate on paper) are actually related and have a common ancestor. And no, we don’t have to go digging up g-g-g-g-grandfather John to get his DNA – remember, he passed it on to his son, and his son’s son, and so on to the present day. So we find a living male descendant and they get their Y-DNA analysed. That gives us a pretty good indication of the genetic signature for the whole family tree (well, the male side, anyway). If we want to check it, we find a distant cousin of the first person who tested and get their DNA analysed. If its a very close match, then bingo – we know the Y-DNA genetic signature (and whats known as a haplogroup) for our whole family group.

And that’s where it starts getting interesting, because then we can start comparing that genetic signature to the results from other men who share the same surname but don’t appear on our family tree. If the matches are close, then we can start thinking “is there a common ancestor for these families another generation or two back? Are the families linked in some way?”. Depending on what we already know about either family group, it can help target the paper research to get those families linked, or take us back another generation or two.

So, back to the example of the two men (lets call them John and James) we think might be brothers: We find two g-g-grandsons (or any male descendant in a direct father-son line) of each, preferably the most distant cousins we can find, and get a Y-DNA test done for all four men. Usually a 37 marker Y-DNA test is a good place to start. This looks at 37 sections of each persons DNA that the genetic scientists think are most useful for our purposes (the bits that are least likely to randomly change over generations). The results come back looking like a fairly meaningless string of numbers which are fairly useless on their own, but allow us to compare each persons result with others in the database of the testing company.

What we would expect to find then in this example is that the two descendants of John show a very close match, and the two descendants of James also show a very close match (because we know from conventional paper based research that they are related). If all four match very closely, then that’s further evidence to add to our theory that John and James were really brothers. Not conclusive proof- but pretty solid evidence.

So what are you waiting for? If your family’s genetic signature hasn’t yet been tested, how about considering contributing to the genealogical record and resource for your family by finding one or two men to take a Y-DNA test. If you are a male S-NN-T descendant then please check out the Sinnott/Sennett (and variants) surname project at familytreeDNA.com – you even get a discounted rate for the Y-DNA37 test if ordered through the project. http://www.familytreedna.com/group-join.aspx?Group=Sennett

If you aren’t a S-NN-T, then there are plenty more surname and geographic projects you can join.

And a final note: be on the alert for surprises in your DNA – sometimes its as simple as realising that what you thought was a surname that had come down through the male line, has actually been taken from a female at some point who kept her maiden name (which means the DNA signature will match the surname of the father of her children, and not the surname the child was given). Sometimes the man who is believed to be the father just isn’t – and that will show by his real sons having a different DNA signature to the ones fathered by another man. Often these NPE’s (non-paternal events) will be many generations back, but they could be much closer.

I hope this inspires you to find out more about genealogical DNA studies and get involved in some way.

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