Because the chromosomes mothers and fathers pass on to their children are reshuffled versions of the ones they inherited from their own parents, it is hard to use most chromosomes to trace genealogy back very far. Most of the Y chromosome, however, is handed down from father to son without changes. Likewise, in humans, the tiny bit of DNA contained in an unusual package of genetic material known as mitochondrial DNA is passed down from mother to child without any recombination.
For this reason, these types of DNA can be used to trace your ancestry.