According to Ugo Perego, senior researcher at Sorenson Molecular Genealogy Foundation, the nucleus of the human cell contains about 3.2 billion pieces of genetic information. This "nuclear DNA" is harder to use for population studies because it changes too quickly from generation to generation -- almost like shuffling a huge stack of cards.
Outside of the nucleus, but still in the human cell, is something called mitochondrial DNA, or mtDNA for short. MtDNA contains only 16,569 pieces of genetic information. Perego said mtDNA is good for tracing populations because, unlike nuclear DNA, it doesn't recombine with other DNA every generation. However, every once in a while, a random mutation -- a slight mistake in copying the mtDNA -- will be passed on to the next generation.