The Theory of Autosomal DNA Matching

You have seen the basic theory behind y-DNA matches and mt-DNA matches. Briefly the Y chromosome changes minutely about every fourth generation, as you can see by comparing 67 marker results from the same male lineage. In actuality it is a matter of statistical probability and the minor change may have occurred just between the last two generations, or it may have been six generations ago. The usefulness of using the Y chromosome for matches is to learn more about a male line of descent. The mitochondrial DNA changes much less frequently. It changes at about 1/10th the rate of changes in y-DNA. Thus it is of less genealogical help when fine-tuning recent generations of a family line, but can definitely establish a connection to an historical maternal line. (For more on y-DNA or mt-DNA see previous Taylor blogs.)


Autosomal DNA which is tested through as “Family Finder” at www.ftdna.com, is entirely different. This test looks at segments of the non-sex determinate chromosomes and compares matching segments throughout the Family Tree DNA database.

Here is the theory behind this test. Each of us has received our genes from our biological forebears. Each time a new human is created, some genes from each parent are combined and some are not included. Approximately ½ of our autosomal genes come from each of our parents. Each of them received half of their genes from each of their biological parents.

To simplify our understanding we are going to consider these blocks of genetic material as playing cards. Imagine you are holding a hand of cards—8 cards—and each card represents a unique inheritance from one of your great-grandparents. From Great Grandmother Susie you got the Ace of Spades, for example. But you have 8 cards and one is from each of your great-grandparents.

Let’s follow that Ace of Spades backwards in time. Great-grandmother Susie received it from one of her great-grandparents. And that great-grandparent received it from one of his or hers…

Now back to Great-Grandmother Susie. She was part of a large family. Let’s say there were ten children. Maybe half of those children, Susie’s siblings, got that Ace of Spades in their genetic package. Those of Susie’s siblings who reproduced gave their children part of their genetic heritage. For many it may have included the Ace of Spades, but probably not for all.

Now consider again the hand of cards you received as your birth package of DNA. You got the Ace of Spades (which is a simplified way of saying you got a unique segment of DNA). Anyone else in the Family Tree database who also has the Ace of Spades is a cousin to you. Their relationship may be as far back as seven generations, or more! However we know that you are biologically/genetically related individuals. Now the practical aspects of using this autosomal testing method, Family Finder, become of interest. We will take up that topic in a future blog.

Lalia Wilson for the Taylor Surname Project

Lalia is keenly interested in genetic genealogy for many reasons, among them the many common surnames in her personal genealogy. In addition to Taylor, these include: Jackson, Johnson, Madden, Moore, Robinson, Stone and Wilson. These families were in North America prior to 1700 and followed migration paths from Virginia or Pennsylvania or New York to Ohio then Missouri or to Kentucky then Missouri. Please contact Lalia with questions about the blog, or story ideas, or if you think you’re a relative: Lalia W /at/ aol dot com.