With the discovery that a person's DNA contains information that has been passed down relatively unchanged from our earliest ancestors, analysis of DNA is just beginning to be used for genealogical research. There are two DNA types of particular interest. One is the mitochondrial DNA which we all possess and which is passed down with only minor mutations through the female line. The other is the Y-chromosome, present only in males, which is passed down with only minor mutations through the male line.
Genealogical DNA Test
A genealogical DNA test involves examining the nucleotides at specific locations on a person's DNA. The test results have no medical value and cannot determine genetic diseases or disorders; they are only used for genetic genealogy.
How a test is done
A genealogical DNA test is generally done by taking a painless cheek-scraping at home and mailing the sample to a genetic genealogy laboratory for testing. Some laboratories use mouth wash or chewing gum. Certain laboratories store DNA samples for ease of future testing whereas others destroy them, guaranteeing that a sample is not available for further analysis.
Types of tests
Genealogical DNA tests allow one to identify their recent and far distant ethnic and geographic origins. Most commonly, one uses Y chromosome (Y-DNA) testing and mitochondrial DNA (mtDNA) testing. Y-DNA and mtDNA tests are by far the most common and often people refer to them as the only genealogical DNA tests. However, additional tests are available for distant and recent ethnic origins.
Y chromosome (Y-DNA) testing
A man's paternal ancestry can be traced using the DNA on his Y chromosome (Y-DNA). Women who wish to determine their paternal ancestry can ask their father, brother, paternal uncle, paternal grandfather, or a cousin who shares the same paternal lineage to take a test for them (i.e. any male family member who has the same surname as her father).
What gets tested
Y-DNA testing involves looking at segments of DNA on the Y chromosome (found only in males) where sequences of nucleotides repeat, known as short tandem repeats (STRs). These segments are considered "junk" DNA. The segments which are examined are referred to as markers and are designated by a DYS number (DNA Y-chromosome Segment number).
Understanding test results
A Y-DNA test will look at 10-43 markers on the Y chromosome. The results will tell how many repeats the test subject had at any given marker; the variations of repeats are known as alleles. For example, at DYS455, the results will show 8, 9, 10, 11 or 12 repeats (source). The test results are then compared to another person's results to determine the time frame in which the two people shared a most recent common ancestor (MRCA). If the two tests match on 37 markes, there is a 50% probability that the MRCA was less than 5 generations ago and a 90% probability that the MRCA was less than 17 generations ago.
Records in genealogical research
Records of persons who were neither royalty nor nobility began to be taken by governments in order to keep track of their citizens. (In most of Europe, for example, this started to take place in the 16th century.) As more of the population began to be recorded, there were sufficient records to follow a family using the paper trail they left behind.
As each person lived his or her life, the major events were documented with a license, permit or report which was sent to a local, regional or national office or archive. A genealogist locates copies of these records, wherever they have been stored, and rearranges the information about each person to discover family relationships and recreate a timeline of each person's life once again.