There are many different DNA tests available depending on the purpose of your research.
Tests for Genealogy
Genealogists use two main DNA tests: the Y-chromosome (for male lines) and the mitochondrial (for female lines). All men get the Y-chromosome of their father with no or small changes, and pass it on through their male children. Any male cousins by a common male ancestor through direct male lines will share common DNA.
When you get a Y-chromosome DNA test, the chemical markers in certain parts of the Y-chromosome are analyzed. There are two main parts of the test. One part tests a marker that does not change over time, establishing the haplogroup of the testee. There are 28 haplogroups among all human beings. Other markers that are tested change more rapidly over time and give the testees haplotype. The combined use of the haplogroup and the haplotype for an individual establishes a signature for the person as well as distinguishing between separate direct male lineages.
Mitochondrial DNA is passed from a mother to all her children. The female children then pass it to their children. It is important to note that mitochondrial DNA does not mutate at a high rate like Y-chromosome DNA. Because of this, mitochondrial DNA is not useful to genealogists. Mitochondrial DNA is used to study the movements and developments of large populations over a long period of time.
Tests for Recent Relationships
The most common and most recognized DNA test used for recent relationship testing is the paternity test. Paternity testing reveals whether a man could be the biological father of a child.
A maternity test is just like a paternity test but reveals whether a person is the child of a certain woman. Maternity testing is often used to resolve baby mix-ups in a hospital nursery or to confirm that an in vitro embryo was implanted into the right mother. Both paternity and maternity tests are used to confirm that someone has been reunited with his birth parents.
Different tests are used to determine parentage if the father is not available for testing. A grandparentage test determines if someone is the biological grandparent of a child.
A genetic reconstruction involves the testing and comparison of indirect relatives, like aunts, uncles, and cousins, to prove the person who is the biological father of a child. Genetic reconstruction requires the availability of the mother of the child.
A siblingship test is used to determine if two people share one or more of the same parents. Siblingship tests are possible without the availability of the mother, but cost more and take longer without her participation.
A twin-zygosity test is used to determine if siblings are fraternal or identical twins. In general, doctors can tell at birth what kind of twins siblings are. However, if this information is not known, it will reveal the truth. Knowing whether twins are identical or fraternal can be very important in certain heath situations such as an organ or tissue transplant.
These tests are used in cases such as Social Security benefit proof, inheritance claims, and immigration cases.