How is DNA used in genealogy?
A1. We compare the DNA of two people to see if their samples match and, if they do, those two people may be related. If their DNA does not match, we know they are not related. All of your DNA came to you from your parents as an identical copy of their DNA — half from your mother and half from your father. Most of it is a random “jumble” of fragments from your mother and father. This jumble means we can’t tell who contributed which part and it is of little use in tracing ancestry. This jumbled DNA is called “autosomal” DNA. There are two types of DNA which are not jumbled up when passed to the child. Each of them comes from only one parent and they are virtually identical to that of the parent and to their parent. One type comes from the mother and is called mitochondrial DNA (abbreviated mtDNA). The other type comes from the father to his sons and is called Y-chromosome DNA (abbreviated Y-DNA).
The sperm and egg cells each contain one of each kind of chromosome, which may be the one inherited from the mother or that from the father. Since there are two possibilities for each of the 23 chromosomes, there are 223 different possibilities for an egg or sperm cell; this is more than 8 million. In fertilization, one of the 8 million possible sperm cells combines with one of 8 million possible kinds of egg cell, resulting in an enormous number of possible chromosome combinations in the fertilized egg. Furthermore, in the process of cell division leading to the formation of sperm and egg cells, the chromosome pairs within the man and the woman may exchange bits of themselves with their pair-partners, resulting in even more variety in the chromosomes of the resulting child. This enormous variety accounts for the essential uniqueness of each person, and of that persons DNA.
