Common disease-common variant

The common disease-common variant (often abbreviated CD-CV) hypothesis predicts common disease causing alleles will be found in all populations. Variations in the human genome in the form of single nucleotide polymorphisms (SNPs) are common in independent populations. Common variations are concentrated in coding and regulatory sequences in genes. Evolutionary neutral diseases, complex polygenic diseases, are caused by these variations. Each variation in a complex disease will have a small effect on the disease phenotype resulting in additive or multiplicative effect of many susceptibility alleles. The hypothesis has held true in the case of putative causal variants in the apolipoprotein E, APOE ε4, associated with Alzheimer's disease. IL23R has been found to be associated with Crohn's disease; the at-risk allele of has a frequency 93% in the general population.