DNA contains the genetic instructions that determine hereditary traits in humans. It exists in nearly every cell as a double-stranded molecule that consists of a sugar-phosphate backbone with nucleotide bases attached. The four nucleotide bases are adenine, guanine, cytosine, and thymine. DNA is packaged into chromosomes, which are found in the cell nucleus. Chromosomes determine an individual's sex, with females having two X chromosomes and males having one X and one Y chromosome. The Y chromosome is only passed from father to son and determines maleness. DNA testing analyzes markers on the Y chromosome, which are sections of DNA that vary in the number of repeated sequences between individuals. This allows DNA tests to determine family relationships along the