Each copy of the human genome is made up of just over 3.2 billion pairs of nucleotides (the structural units of nucleic acids), comprising the individual “letters” that make up all deoxyribonucleic acid (DNA) sequences. The sequence is housed on 23 chromosomes, containing approximately 22,000 genes as well as the regulatory switches that turn those genes on and off. Every person carries two copies of the genome, one inherited from each parent, in each cell in the body (excluding the gametes). As the genome is passed from one generation to the next, changes occur in the DNA sequence at a very low rate; over time, some of these changes are lost, while others remain.

As a result, the genomes carried by present-day humans contain a large number of genetic variants that arose as new mutations many generations ago. These old genetic variants are now shared across many genomes, making them common in the human population. These common variants may influence the risk of common human diseases and other traits, an idea that is referred to as the common disease–common variant (CDCV) hypothesis. Whole genome (or genome-wide) association studies (WGAS) are designed to test the CDCV hypothesis by examining the effect of common genetic variants on the risk of human disease, response to drug treatment, and other human traits.