This chapter reviews the basic principles of human genetics to serve as a basis for other studies that deal with specific genetic approaches in clinical research. Genetics is the science that deals with the storage of information within the cell, its transmission from generation to generation, and variation among individuals within a population. Human genetics research has a long history, dating to the study of quantitative traits in the nineteenth century and to the study of Mendelian traits in the first decade of the twentieth century. Medical applications have included such landmarks as newborn screening for inborn errors of metabolism, cytogenetic analysis, molecular diagnosis, and therapeutic interventions such as enzyme replacement. Medical applications historically have been limited to relatively rare disorders caused primarily by mutations in individual genes or structural abnormalities of chromosomes. Recent advances, and especially the sequencing of the human genome, have opened the possibility of understanding genetic contributions to more common disorders, such as diabetes and hypertension. Genetic approaches are now being applied to conditions in virtually all areas of medicine. Genetic information is stored in the cell as molecules of deoxyribonucleic acid (DNA). Each DNA molecule consists of a pair of helical deoxyribose-phosphate backbones connected by hydrogen bonding between nucleotide bases. There are two types of nucleotide bases, purines (adenine [A] and guanine [G]) and pyrimidines (cytosine [C] and thymine [T]). © 2009 Copyright © 2009 Elsevier Inc. All rights reserved.