There are three basic types of DNA tests
Most of your DNA is autosomal/chromosomal DNA. Half your autosomal/chromosomal DNA is from each of your parents. Each generation these halves are put together in a blender and mixed together into a pattern unique to only one individual in a population. Because of this uniqueness, autosomal DNA tests are useful for forensics and due the popularity of shows like CSI, are the kind most people have heard of. Unfortunately for genealogists, because this DNA is "all mixed up" (recombined) from both parents, it cannot be isolated for genealogical information.
There are two types of DNA that are useful for genealogists because they pass almost identically from one generation to the next. Y-DNA only passes from father to son, and mitochondrial DNA (mtDNA) only passes from mothers to her children. Most mt-DNA changes more slowly than Y-DNA over time, and so is more often used for anthropological studies of deep genealogy such as examining the ancient movements of peoples across continents. One can know via mtDNA that one shares a common ancestor, but that ancestor may have lived anywhere from 100 to 5,000 years ago. Unlike mtDNA, portions of Y-DNA undergoes subtle, but more rapid changes within a few generations, and so it is possible for a particular ancestor to have a DNA signature so unique that later generations of male descendants can identify with high certainty that they shared a common ancestor within a few generations when the common ancestor lived.
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.
Closer look: Y-DNAEdit
Y-DNA tests for the genetic composition of 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 will show virtually identical Y-DNA test results. A father and daughter would not be comparable through Y-DNA testing since the daughter does not carry the Y chromosome. 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"!
Every few generations, it is likely there will be a slight change in so called "junk dna" portions of Y-DNA. Without these changes, the Y-DNA of all males would be identical. Many of these changes are rapid, occurring in the "non-coding ("junk") portion of the DNA. As a result, every few generations there are some gradual changes passed down from father to son. Those changes gradually build up through time, and create a unique signature. These changes are identified via small segments of the dna called markers, and more technically referred to as Y-STR markers. Each marker has unique and intimidating technical number assigned them such as DYS 393. It is these markers 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.
Various companies offer these tests and what gets measures is the proportions the individual has of each of these markers. The numbers look like a long safe combination, eg 14-24-11-12, but a combination that only you and your true ancestor males share. Each number actually stands for how many times the marker in that position repeats. At one time 4 markers were used but today such a small number is laughable. Typical use is now 25 markers, with some companies offering as many as 67 markers. In most cases, 25 or even 12 markers is sufficient for preliminary tests.
It can be an immense time saving value- for example in suddenly acquring 450 relatives due to an exact match, or in saving one from spending years exploring a blind alley. One example concerns an individual had located only 4 other families in the world who shared her surname. She was prepared to do decades of research to identify her connections, but a low marker dna test confirmed that it was impossible she was related to any of them.
By comparing Y-DNA results from multiple individuals known to be actual descendents, it is possible to identify a Y-DNA signature for that ancestor. This means that it is possible to use that signature for purposes of verifying other individuals. Inexact matches for some markers is permissable, since it is known that certain markers mutate at faster rates, so a descendant's signature may be slightly different amongst these markers, especially if several generations have passed So long as none of the slower mutating markers have changed, then there is a very high likelihood that the descendant had a common ancestor with the individual whose signature has been identified.
Closer look: mt-DNAEdit
Mitochondrial DNA (mt-DNA) testing utilizes a different piece of the genetic package. We all, men and women, have mt-DNA. This particular 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. That is, 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 mtDNA signatures last a thousand years as opposed to a hundred years for Y-DNA sequences. Besides also costing much more than Y-DNA tests, because it follows the female line you do not have that nice place-marker of the surname to guide you research. Given current technology, these factors makes mtDNA considerably more difficult to employ for genealogical research.
Drawbacks of such markersEdit
The previously mentioned mutations for Y-DNA markers occur to rapidly changing segments in the so called "junk DNA" sequences. Due to the ease with which such markers change (often they are regulatory sequences that respond to environmental conditions), it is possible for such mutations to regress, and for indentical sequences to develop from completely unique lineages. Longer sets of markers (some companies offer up to 67) make such identical sequences statistically rare for the purposes of genealogy, but their quick mutability makes such markers unreliable for time periods measured in millenia.
Paleoanthropological use of DNA analysisEdit
Rarely, there is a kind of mutation to the stable portions of mt-DNA or Y-DNA in the very stable portions of the coding sequences of the DNA. Since they are in the coding (non "junk") portion, such a mutation must be neutral- that is, it must not negatively affect the survival chances of the child. In such cases, the change is not dangerous- as if a portion of the dna changed from the meaning "the weather will be nice today" to "the weather will be pleasant today". If the substitution is at most "the weather will be warm today", then the mutation is "silent" and because it is in the mtDNA or Y-DNA, it will spread stabily to children. Such mutations are technically known as unique event polymorphisms(UED), and are used to trace migrations across continents. Segments of a population can be identified by whether they care these mutations or not. For example, nearly the entire world's population carries the M168 mutation which occurred 50,000 years ago. Such groupings are known technically as haplogroups and carry the same DNA signatures.
- Trace Your Roots with DNA: Using Genetic Tests to Explore Your Family Tree by Megan Smolenyak , Ann Turner. ISBN: 978-1594860065
- Reflections of Our Past: How Human History is Revealed in Our Genes by John H. Relethford. ISBN:978-0813339580
- News in Genetic Geneology at MSNBC
- ^ Trace Your Roots' pp. 76-77