DNA

What is DNA?

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms.  Nearly every cell in a person's body has the same DNA.  Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).
Mitochondria are structures within cells that convert the energy from food into a form that cells can use.

How does DNA support Genealogy?

Genetic genealogy is the use of genealogical DNA tests, i.e., DNA profiling and DNA testing in combination with traditional genealogical methods, to infer biological relationships between individuals.
On-line services can expand your pool of known living relatives and through their research increase your ancestral knowledge.

Types of tests

There are three major types of genealogical DNA tests:

Autosomal (which includes X-DNA).
Autosomal DNA tests look at chromosome pairs 1-22 and the X part of the 23rd chromosome.  The autosomes (chromosome pairs 1-22) are inherited from both parents and all recent ancestors.  The X-chromosome follows a special inheritance pattern, because females (XX) inherit an X-chromosome from each of their parents, while males (XY) inherit an X-chromosome from their mother and a Y-chromosome from their father (XY).  Ethnicity estimates are often included with this sort of testing. 

Y-DNA looks at the Y-chromosome, which is passed down from father to son.  Thus, the Y-DNA test can only be taken by males to explore their direct paternal line. 

mtDNA looks at the mitochondria, which is passed down from mother to child.  Thus, the mtDNA test can be taken by both males and females, and it explore one's direct maternal line. 

Y-DNA and mtDNA cannot be used for ethnicity estimates, but can be used to find one's haplogroup, which is unevenly distributed geographically.