One fine day (after watching a TV program about descendants of President Thomas Jefferson) I decided to bite the bullet and send in some of my white blood cells to be analyzed for maternal mitochondrial DNA. Before I go into that, a bit of background. As you probably already know, your complete set of genetic information is encoded within 23 pairs of chromosomes in the nucleus of your cells. (Not all cells have a nucleus, however. Red-blood cells, for instance, have no nucleus.) A chromosome is a single piece of coiled DNA, a bio-molecule that holds the blueprint for how living organisms are built. About 99% of all DNA (deoxyribonucleic acid) in your body is found in your chromosomes. DNA consists of a 64-letter (codon) alphabet that spells out your genetic code. The letters or codons are organized into words and sentences called genes―a segment of DNA passed down from parents to child that confers a trait to the offspring. Humans have 25,000-30,000 genes, usually in pairs (one from each parent). Part 2 tomorrow.