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==Introduction==
Humans have 23 pairs of chromosomes which contain their [[Genealogy_Glossary#dna|DNA ]] (deoxyribose nucleic acid). Each of us inherits half of each chromosome from one parent and the rest from the other parent. From the mother one also inherits a piece of genetic information that is contained outside the nucleus of the cell. This genetic material is not part of a chromosome and is called mitochondrial DNA (mtDNA) and is passed nearly unchanged from a mother to her child. 22 of the chromosomes are called autosomes, the 23rd chromosome is referred to as the sex chromosome. For females the sex chromosome contains 2 X chromosomes one from each parent. A male inherits an X chromosome from his mother and a Y chromosome from his father.
At each generation a child's chromosomes are formed by a random combination of their parents 23 chromosomes. The 22 autosomal chromosomes are formed of a random combinaton of the parents DNA. This process tends to copy segments rather than individual locations while discarding the unchosen DNA. This is how 2 siblings will share DNA but not have identical copies with one another but yet have significant identifiable in common. The 23rd chromosome whether X or Y is passed from the donor parent whole and is not subject to this recombining process. In a male tests can distinguish the X and Y parts of the chromosome thus identifying genetic material from the mother and father. In a female the test can identify the 2 separate Xs in the chromosome but cannot identify which was donated by each parent.