What are genes that mask other genes?

http://wiki.answers.com/wiki/Epistasis refers to genetic interactions in which the mutation of one gene masks the phenotypic effects of a mutation at another locus.^{http://wiki.answers.com/#cite_note-0} Systematic analysis of these epistatic interactions can provide insight into the structure and function of genetic pathways. By examining the phenotypes resulting from pairs of mutations we begin to understand how the function of these genes intersects. Genetic interactions are generally classified as either Positive/Alleviating or Negative/Aggravating. In the case of a positive epistatic interaction, the double mutant exhibits a phenotype which is neutral or improved relative to the phenotype of a single mutant ^{http://wiki.answers.com/#cite_note-1}. This phenotypic response occurs when both genes lie within the same pathway. Conversely, negative interactions are characterized by an even stronger defect than would be expected in the case of two single mutations, and in the most extreme cases (synthetic sick/lethal) the double mutation is lethal. This aggravated phenotype arises when genes in compensatory pathways are both knocked out. High-throughput methods of analyzing these types of interactions have been useful in expanding our knowledge of genetic interactions. Synthetic genetic arrays (SGA), diploid based synthetic lethality analysis on microarrays (dSLAM), and epistatic miniarray profiles (E-MAP) are three important methods which have been developed for the systematic analysis and mapping of genetic interactions. This systematic approach to studying epistasis on a genome wide scale has significant implications for http://wiki.answers.com/wiki/Functional_genomics. By identifying the negative and positive interactions between an unknown gene and a set genes within a known pathway, these methods can elucidate the function of previously uncharacterized genes within the context of a metabolic or developmental pathway.