Deoxyribonucleic acid (DNA) contains chemical instructions which control the physical traits of a living organism. DNA is passed from parent to offspring during reproduction and thus DNA is the vehicle of physical inheritance.
There are two distinct classes of DNA.
Mitochondrial DNA (mtDNA) is found in cellular structures called mitochondria. It is passed from generation to generation only through the female line since it is not found in sperm (the male contribution).
Nuclear DNA is found in the nucleus of every cell and consists of a contribution from both the mother and father of the individual. This nuclear DNA is found in pairs of 22 chromosomes which are joined during reproduction.
In addition there is a set of sex chromosomes, X and Y, which are paired in females (XX), but are an unmatched set in males (XY). Y- Chromosome DNA or Y-DNA is the most stable chromosomal DNA because it is never paired with another of its kind, it is cloned.
Mitochondial DNA is shared by all maternal siblings but is passed on the next generation only by daughters. Mitochondrial DNA analysis yields information on the genetic affinity of the line consisting of the mother's mother's mother, etc.
Y chromosomal DNA is shared by all male siblings with a common father. It is passed on only to sons and thus follows the paternal surname naming pattern now common in European cultures and their derivatives. If there has been no interuption of the passage by adoption, illegitimacy or surname modification, males with a surname in common will often have a common ancestor in their pedigree. For this reason there has been much interest in Y chromosomal DNA by those trying to establish a link to others with the same surname.