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Epistasis, first defined by the English geneticist William Bateson in 1907, is the masking of the expression of a gene at one position in a chromosome, or locus, at one or more genes at other positions. Epistasis should not be confused with dominance, which refers to the interaction of genes at the same locus. The human genome contains from 30,000 to 70,000 gene loci. Some of them are involved in numerous interactions, making it difficult to identify their role in development and metabolism. As we learn more about the human and other genomes, it becomes clear that the borrowed phrase "no gene is an island" is an appropriate expression to describe the interplay among gene loci.