How DNA Helps with Genealogy

I've received some questions about DNA testing and how we can learn about ancestors from generations ago by having someone who is alive today take a DNA test. Following is 'my' understanding of how it all works. If I've got it wrong, please send me a message to let me know. But here goes...

The way DNA testing for genealogy works is that all the people who get tested submit background information along with their saliva. The results of their test are recorded with the information submitted. For example, I would say that my mother was from Holland and that my father was English/Irish/Scottish. So that information would go into the database along with my DNA markers. When people submit their DNA plus their background information, that creates a database that can correlate specific markers with specific ethnicity, regions of the world, etc. The bigger the ’sample’ of the population, the more accurate the information about ancestral heritage as it relates to DNA.

But just like on TV, the DNA markers can also be compared directly with other living people. If your markers match someone else’s, then you are related. The more markers you have in common, the closer the genetic relationship. Once you know you are related, you can then look at their ancestry if they’ve provided it.

There are several types of DNA testing for genealogy: Y, mtDNA (also called mitochondrial) and autosomal. Y and mtDNA are both related to gender: Y to male and mt to female. These markers represent only a small portion of the DNA in a person.

Autosomal covers all the DNA other than the gender-related markers. Autosomal markers represent all the DNA that your body has combined from your parents to make the unique you. There are many more autosomal markers than gender-related ones, and, as a result, autosomal is more accurate, especially in comparison with close relations. But the gender-related markers have specific use.

The Y-chromosome doesn’t change as it is passed from male to male except for small genetic mutations. It does not re-combine like our other DNA does. Technically, the Y-chromosome in a living male could be traced back to the first male human being and be nearly identical. So all the males who are alive today have essentially the same Y-chromosome that their male ancestors had hundreds, even thousands, of years ago. It’s unique that way, which is why I was willing to pay the big bucks ($400+) to have my brother's DNA tested, since he’s the only male we have left in our direct family.

Lots of people in the Y-database haven’t provided a family tree. Some just want to know their ethnicity; others are adopted and are looking for families and so they don’t know anything about their ancestry; still others simply have no interest in sharing that information. Luckily, our closest match in the database had a family tree attached, so I was able to look at the earliest ancestors listed to see where they lived. That happened to be Scotland. A few other matches also had trees, and their ancestors were also all from Scotland.

Because it is so expensive, however, there aren’t nearly as many people taking that Y test as there are who are taking a regular autosomal test (Ancestry DNA, for example). As a result, the database and ’sampling’ of the population for Y-DNA is much smaller, which means you are less likely to find close genetic matches than you are if you did an Ancestry DNA test.

Millions of people have done Ancestry autosomal tests, and more and more are getting these done every day. So the autosomal databases are huge. Because of this, not only are you more likely to find fairly close living relatives (second cousins, for example), but the ethnic information you receive from the test is more detailed since the sampling of the population is much larger. Ancestry upgrades its algorithms regularly to account for all the new information being added to the database. So as time goes on, the information you receive from an autosomal test will be updated and should become even more accurate.