A genealogical DNA test involves examining the nucleotides at specific locations on a person's DNA. The tests results are meant to have no informative medical value and do not determine specific genetic diseases or disorders they are intended only for use in genetic genealogy.
The general procedure for taking a genealogical DNA test involves 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 instead of cheek swabs. Most of the laboratories offer to store DNA samples for ease of future testing. All laboratories will destroy the DNA sample upon request by the customer, guaranteeing that a sample is not available for further analysis.
Types of tests
Genealogical DNA tests allow one to compare their DNA with that of others. They are also used to identify possible recent and far distant ethnic and geographic origins. The most popular such tests are Y chromosome (Y-DNA) testing and mitochondrial DNA (mtDNA) testing. Other, less well validated tests attempt to determine 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). This is particularly useful because the Y chromosome, like many European surnames, passes from father to son, and can be used to help study surnames. 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 genetic markers and are designated by a DYS number (DNA Y-chromosome Segment number). These STRs may also indicate a likely haplogroup for the Y chromosome, though this can only be confirmed by specifically testing for that haplogroups' single nucleotide polymorphisms (SNPs).
Understanding test results
A Y-DNA test will look at between 10 and 59 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 markers, 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.
Mitochondrial DNA (mtDNA) testing
A person's maternal ancestry can be traced using his or her mitochondrial DNA (mtDNA). The DNA in the mitochondria (an organelle inside most cells) is generally passed down by the mother unchanged, though some exceptions have been shown. All test results are compared to the mtDNA of a European woman in haplogroup H, which is known as the Cambridge Reference Sequence (CRS). Any "mutations" or "transitions" that are found are simply differences from the CRS. The test results are compared to another person's results to determine the time frame in which the two people shared a most recent common ancestor (MRCA).