Schematic diagram to illustrate recombination. minimally technical background for physicians to understand the changes that will be occurring in the advancement and practice of medicine. Because most practicing physicians lack a formal background in modern molecular genetic analysis, the promise of SNP mapping will be presented here in basic terms. More technically detailed reviews are certainly available, but the focus in this article will be on SNP mapping as it relates to the practice of medicine. 2 How do researchers find genes and variations of those genes that cause or predispose individuals to disease? The human genome contains 3 billion nucleotide bases and approximately 100,000 genes. It is just now possible to take DNA from patients with and without a disease, determine the order of all the bases (called the sequence) in hundreds of patients and controls, compare the sequences, and find the genes and gene variants that lead to disease.(The 15-year Human Genome Project is determining one reference human genome sequence.)

Linkage studies that use identifiable landmarks throughout human DNA are used to identify inherited genetic mutations that cause particular diseases. 3 These studies are based on a fairly simple concept. Suppose that the human genome is considered as a single long ribbon with numerous bands of colors (eg, red, blue, green, brown); suppose, too, that the different ribbons that we all have can be stretched out. Along the ribbons one would see that they are almost identical, with the same colored bands. However, certain bands along the ribbons would have different colors. The position and nature of the easily identified differences could be