DNA test

This is a short note on the use of DNA testing. It is a "starter" for a longer more complex article.

The Quick Summary
There are two basic types of DNA tests

a) Y-DNA b) mt-DNA

There are other more specific types of DNA tests that can be performed, but they are largely special purpose tests, of interest primarily to a relatively limited population. The Y-DNA and mt-DNA tests are universally applicable.

Y-DNA tests for the genetic compostion on the male or Y-chromosome. That means that you can get only Y-DNA test results for men. Y-DNA testing is especially useful for tracing surname lines. A father and son should show virtually identical Y-DNA test results. A father and daughter would not be comparable through YDNA testing. Because a fathers and sons Y-DNA test results are virtually identical, the results of a grandfather and grandson would also be "virtually" identical. That "virtual" identicalness continues back indefinitely through the male ancestor line back just about as far as you want to go...with the important exception that there is  significance to that word "virtual"!

At every generation there is a slight change in Y-DNA through natural mutations. As a result, there are some very gradual changes in YDNA along the line of transmission. Those changes gradually build up through time, and so if you get back far enough there are significant and detectable changes in the YDNA between an ancestral male and a descendant. Its those changes that serve as the basis for determining whether two men bearing the same surname have a common ancestor some time in a reasonable genealogical timeframe---say the last three hundred years. If two men with the same surname are tested and have virtually identical Y-DNA test results, then we know that they share a relatively recent common ancestor who presumably also bore that surname. If their results are wildly different, then we know that even though they have the same surname, they do not share a recent common ancestor---and that their individual ancestors adopted the same surname independently of each other.

mt-DNA, or mitochondrial DNA, testing utilizes a different piece of  the genetic package. We all, men and women, have mt-DNA. This parituclar part of the package is inherited SOLELY from our mothers. Our fathers do not contribute any of their own mt-DNA to their children. If you test a mother and her child (son or daughter) they should show virtually identical mt-DNA.---with exactly the same caveats about what "virtually" signifies---namely that there are very slow natural changes in mt-DNA (just as there are in Y-DNA), and that given enough time, and generations, those changes can a accumulate to a significant degree.

mt-DNA can be used to trace ancestry through the maternal line largely in the same way as Y-DNA can be used to test ancestry through the male line.---The major difference is that you do not have that nice place-marker of the surname to guide you research. In most cultures, children do not adopt their mothers surnames. This makes it considerably more difficult to follow the heritage with mt. DNA.