Genealogical DNA testing is based on testing the Y chromosome. The Y chromosome is passed down only through an unbroken male line, so direct male line descendants are needed.
A molecule of DNA is a long string of nucleotides that are linked together. I'm not sure if the proper terminology for a nucleotide is that it is a molecule or not. But in any case, after the nucleotides are linked together, the string of DNA as a whole is a gigantic molecule.
There are four nucleotides: adenine, thymine, guanine, and cytosine. They are usually just abbreviated as A, T, G, and C. These four letters serve as an alphabet for the genetic code. So a string of DNA may be thought of as a string of letters in the genetic code. Such a string might look something like GATACCATGCGTA... etc.
Most of the DNA molecules possessed by each person consist of a combination of the DNA from all their ancestors, and the DNA from their ancestors is all mixed together. The "all mixed together" description is an oversimplification, but it will do for the purposes of this narrative.
The only DNA that is not from all your ancestors and that is not "all mixed together" is Y-DNA, which is passed down only from men to their sons. And Y-DNA is typically passed down completely unchanged. So if you are a male, your Y-DNA is identical to your father's Y-DNA that is identical to his father's Y-DNA etc. for generation after generation after generation. The only exception is that occasionally there is a minor mutation in the Y-DNA. Were it not for the occasional mutation, every male on earth would have identical Y-DNA.
As compared to geologic time of millions or billions of years, mutation rates in Y-DNA are very fast. So mutation rates in Y-DNA are very fast as compared to the mutation rates that drive evolution. However, mutation rates in Y-DNA are very slow by every day standards. A mutation might occur at a particular place in a string of Y-DNA about once every 500 generations for example. Fortunately, there are enough different places on a string of Y-DNA where we might look for mutations that the mutation rate is fast enough to be useful for genealogy.
The places on the string of Y-DNA where we look for mutations that might be useful for genealogy are called markers. A marker contains a short sequence of DNA such as GATA, and that sequence is repeated a number of times. If GATA were repeated three times, we would have GATAGATAGATA. The short sequences such as GATA that are repeated are called alleles. A mutation that is useful for genealogy consists of a change in the number of repetitions for the allele.
If two men are related, Y-DNA testing can not determine with certainty how far in the past their common ancestor might have been. Testing can only provide a statistical estimate. But again, the more markers there are that are tested, the more accurate the statistical estimate can be expected to be. Also, as more research is done, more accurate estimates for the average mutation rates of various markers can be calculated, and markers with faster average mutation rates can be found and used.
This page last edited on 02 Sep 2010.