As a co-administrator of a y-DNA/surname project, I and my colleagues get many questions about what various genetic tests can do for people. The long answer probably requires an advanced degree in genetics… something most participants don’t have. Here is a simpler explanation which is true in the macro sense, though extended details are not covered.
Basically, there are three different kinds of genetic tests used in genealogy. One test is for markers on the y-chromosome. This test requires a sample from a male, as females do not have a y-chromosome. It looks at markers and allows you to compare them with other samples of known or unknown genealogies in hope that you will find a relative who proves a specific lineage.
The second type of test concerns mitochondrial DNA, the DNA which was in the egg from your mother (that became you!) and consequently the bodies of your cells today. Mitochondrial DNA comes only from your mother. It is only passed down a female line. Your mitochondrial DNA came from your mother and her mother and her mother back in time to the first human woman.
The third test is of autosomal DNA. Autosomal DNA is the DNA other than the sex chromosomes. In reproduction the autosomal DNA is mixed up with the creation of each new child, sometimes coyly referred to as a “transmission event.”
These three types of tests give us different information; and importantly, that information yields different genealogical information for different time periods.
Let us begin with our mothers and mitochondrial DNA. Mitochondrial DNA changes very slowly. One research paper recently reported a rate of one change per 371 “transmission events.” Thus, over 371 generations, there is a 50% chance that one child will have a change in her/his mitochondrial DNA. Population geneticists and genealogists define a generation as somewhere between 20 and 30 years. If we use 25 years here, we are talking about the likelihood of one change per 9,275 years. Now this change is random, so it could have occurred between you and your mother, but the likelihood of that is 1/371.
Here’s the simple way to look at this. Suppose we represent your mother’s mitochondrial DNA (mt-DNA) as a hand of four playing cards: the 2 ♦, the Queen ♥, the 10 ♠, and the Knight of ♣. The odds are that your mt-DNA is exactly the same (the 2 ♦, the Queen ♥, the 10 ♠, and the Knight of ♣); actually 370/371, or 99.73%. And if there is a change, it is minor and closely related. An example of a change would be represented by this configuration: the 2 ♦, the Queen ♥, the 9 ♠, and the Knight of ♣. We expect this same situation to hold true up and down your maternal line. A fifth cousin sharing the same maternal lineage will likely have the exact same mt-DNA with perhaps one minor change. For that reason, mt-DNA is very useful for establishing maternal shared ancestors, but of much less use to understand lineages in what we call genealogical time—the seven hundred years since surnames and some record keeping became more common. (Though royal lineages have been kept for thousands of years.)
Because mt-DNA changes so slowly, and because it is present in much larger quantities in people than other DNA and usually survives for years after death and burial, it is used to understand the genetics of earlier humans. One group of people killed by the Mount Vesuvius volcanic eruption of 24 August 79 AD (which destroyed the two ancient Roman cities of Pompeii and Herculaneum) was analyzed over 1,600 years later. The mt-DNA showed that six of 13 individuals in one house were maternally related, all having the unusual haplogroup T2b. Because of the slow rate of change of mt-DNA, scientists can confidently say that all of our mitochondrial lineages trace back to a common ancestor who lived in Africa 100,000 to 150,000 years ago. Some lineages migrated out of Africa about 60,000 years ago, while others remained.
I will return with further information about the rates of change for y-DNA and the autosomal DNA. The card game becomes much more complicated! Mitochondrial DNA changes so slowly you could think about dealing out the same four cards 371 times in a row. But for y-DNA (at 67 markers) it is just 3-4 hands in a row. And for autosomal DNA you get a new shuffle and a new hand with every transmission event.
Lalia Wilson for the Taylor surname project