Such interactions between loci probably occur in the genetic etiology of complex traits such as the psychiatric disorders schizophrenia and manic depression. David Lykken, a genetic psychologist at the University of Minnesota, coined the term "emergenesis" to describe multiple gene interactions involved in a specific complex trait. After comparing EEG (electroencephalogram, or "brain wave") data from identical and fraternal twins, Lykken concluded that multiple-level interactions of independent or partly independent genes must be involved.
Epistatic interactions make it difficult to identify loci conferring risk for complex disorders, and they may be a major reason that researchers have made only slow progress in mapping susceptibility genes for complex disorders. To locate interacting loci involved in the genetic origins of complex diseases requires collecting DNA samples from a large number of families where two or more individuals have the disorder. Such large-scale studies are usually difficult to conduct.
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