DNA is the carrier of our genetic information, which passes from generation to generation. Almost every cell in our bodies contains a copy of our DNA. At conception, a person receives DNA from both their father and mother. We each have 23 pairs of chromosomes and for each pair, one was received from the father and one was received from the mother.
The rules of inheritance work like this. You get approximately 50% of your autosomal DNA from each parent, who got approximately 50% of their autosomal DNA from each of their parents, who got approximately 50% of their autosomal DNA from each of their parents, and so on.
Another way to look at this is that your DNA is about:
50% of each parent
25% of each grandparent
12.50% of each great-grandparent
6.25% of each great-great-grandparent
3.12% of each 3rd great-grandparent, etc.
For Genetic Genealogy, two areas of DNA are tested:
Y DNA - a small portion, which is passed virtually unchanged from father to sons, this is the Y chromosome. Testing of this portion of the Y chromosome provides information about the direct male line, which is the father, his father, and so forth back in time.
Women carry only X chromosomes inherited from their female line ancestors. If women want a Y chromosome test, they must have their father, brothers or paternal side uncles tested.
Mitochondrial DNA - mtDNA is passed from a mother to her children (male and female) ,but only females pass on mtDNA. A test of your mtDNA would tell you about your mother, her mother, and so forth, up the direct female line.
In the chromosomes we have our human inheritance and all our biological characteristics are imprinted in them. Nowadays scientists are starting to understand this genetic information “bar code”. The genetic testing results offer a classification into a certain “Haplogroup” (a population group specific to certain region in the world). A great example for this is the National Geographic’s “Genographic Project”.
The implications for family history researchers are indeed immense. For the first time research can go beyond oral tradition and written records, researching further back than ever before. Racial origins can be determined, long-established pedigrees can be disproved, new family links discovered and DNA testing can be used to demonstrate suspected fostering or adoption in a family. DNA of descendants of America migrants can be tested against samples of Europeans with the same surname and positive matches can be used to pinpoint family relationships or at least a region or village of origin.
Finally, when two individuals have a general match on their test results; it is possible for them to estimate a date for their Most Recent Common Ancestor (MRCA).
DNA tests cannot measure crossover patterns between the male and female line, only the "outside" male-male and female-female lineages. In many cases, your outside lines may not be the dominant ones in your genetic makeup. For instance, about 30 percent of American self-considered “African-Americans” have a Y chromosome that originated in Europe and is Caucasian. Many Native Americans could never “prove” their American Indian origins by DNA because the transmission of American Indian genes many times do not fall within the strictly male-male or female-female line.
Regardless of these limitations, genetic testing provides a lot of useful information and as XXI century genealogists we should use all the resources and technologies we have at our disposal.