The Simple Way to Understand Genetic Genealogy Tests, Part 1

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

The Case for Using Family Finder in a Surname Project

Members of the Taylor Project have almost all participated by means of y-DNA testing. You submitted a sample, provided a basic pedigree and the name of your earliest know Taylor ancestor and now (ideally) you are matched with a group of Taylors. Your group descends from a single line and the other group members are your cousins.

In an ideal world, you know your exact relationship with each member of your Taylor group. Knowing this relationship, each of you can now claim your combined paper trail of lineage.

Isn’t that why you have participated in DNA testing?

Well, it doesn’t always work out like that. And you, dear reader, may be one of the people who is an exception to the ideal portrayed above. This is where the Family Finder test can be of great benefit. The Family Finder test looks at autosomal DNA, which is inherited from all of your lines of descent, not just the male-to-male lineage of y-DNA or the female-to-female lineage of mitochondrial DNA. It is a way to investigate all your lines.

I’m writing this because there is a price reduction on this test for current FTDNA members. Take advantage of a major price reduction through June 22nd.

What can the Family Finder test contribute? Let’s look at several situations.

First, you are a male Taylor, you have a fairly close match with other Taylor project members, enough to form a group, but you don’t know your exact relationship. If you and others in your group do the Family Finder test (henceforth FF), you can close in on your relationship. For example, if your y-DNA suggests a 50% likelihood of a common ancestor within 6 generations, doing the FF will indicate if your common ancestor is within seven generations (the equivalent of 5th cousins) and suggest the number of generations between you. This should focus on exactly where to look for the paper trail indicating a common ancestor. It also may help identify a relative who has not yet tested who would be “the missing link.”

Second instance, you are a female Taylor. You don’t have any y-DNA, and you don’t know any living male Taylors of your line. Doing the FF test will allow you to match with Taylor males currently unknown to you. You can then use their Taylor group, from their y-DNA results, to work towards your earliest Taylor ancestor.

Third instance, you are a male Taylor. Your y-DNA did not match any Taylor line, but did match another surname project. The evidence is pretty conclusive that one of your male ancestors carried the name Taylor, but was not biologically a Taylor. However, you do not know when that NPE (non-parental event) occurred. Was it in your generation? Five generations? More? With the FF test, you can compare your results with other known Taylor relatives, second cousins, great aunts and uncles. If you match them, the likelihood is that the NPE occurred prior to the births of all those tested, and that a common ancestor of all of yours came subsequent to the NPE. (However this inference cannot extend beyond seven generations due to limitations of the testing, nor can it extend beyond the parameters of your individuals sampled.)

Fourth instance, you are a male with a non-Taylor surname. Your y-DNA matched the Taylor line in multiple close matches. You don’t know when the NPE occurred. You do the FF test. If you match a Taylor group with the FF in addition to y-DNA, this suggests the NPE occurred within the last seven generations. It may be fewer generations depending upon the FF results. At least you can begin to discover your roots.

Finally, what about the unique individual who has tested 67 y-DNA markers and mt-DNA and has no matches? I know one of these people. We hope with the FF test to finally find someone who matches!

I look forward to us all finding out our ancestry.

Lalia