Chromosomes are structures in the nucleus of the cell that contain the DNA or hereditary material which form genes. Genes are the commonly known units of heredity, and some may contribute to a tendency toward addiction in ways that are not yet understood. Each chromo-some is an elongated structure that is clearly visible during cell division. Humans possess twenty-three pairs including the sex chromosomes. A male has an X and a Y sex chromosome, whereas a female has two X sex chromosomes. One of each pair comes from each parent.