A relatively rare phenomenon when recessive alleles of different gene loci fail to be complementary in the heterozygote despite the presence of a dominant allele at both loci. Such a situation occurs when the two gene loci encode physically interacting products. The mechanism may involve a particular dosage problem. Reduced dosage at one locus still supports the wild phenotype but simultaneous reduction of dosage at another gene may not permit wild expression of the wild phenotype. Such may be the case when a ligand and its receptor mutate the same time. An alternative mechanism may be based on poisoning the expression of one mutation by another, e.g., one of the locus controls polymerization of another product. The first mutation is innocuous as long as the second locus is fully expressed, but decrease of the level of the product of the second gene may have deleterious consequences. In fact, physical interaction of the products of the two loci is not a requisite for the phenomenon although, most commonly, physical interaction occurs between the two proteins. allelism test, allelic complementation; Yook KJ et al 2001 Genetics 158:209.
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