The allelic spectrum of a disease refers to the number of places in the genome in which DNA sequence variation affects a specified disease, and the relative frequencies of these individual alleles in a population or sample. Most variants will be relatively rare compared with other alleles at their respective location and most typically will have individually weak effects on the risk of the disease. Because variation arises locally by mutation, and spreads only slowly, and because most variants are due to recent mutation, the predominant part of the spectrum consists of many rare variants. However, for all but the simplest of diseases, the effect of a variant, and hence its presence in the disease's allelic spectrum, is dependent on the individual genomic and environmental contexts in which it is found.

Key Concepts