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Genetic redundancy masks the gene functions (eg it masks diverse functions of the tumor suppressor gene). I have read on the internet that gain of function alleles can solve a such problem caused by genetic redundancy and would like to know how.

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Genetic redundancy can be a problem when studying knockout or hypomorph mutations of genes, or analogues such as exogenous RNAi. The redundancy can consist either in incomplete knockout (the genome contains more than one copy of the gene but only one copy is mutated) or functional redundancy (different genes doing more or less the same thing). If you try to figure out what a gene does using a loss-of-function allele, the other copies or redundant genes may weaken or completely mask any phenotypic manifestation of the lost gene function.

Gain-of-function alleles can be useful in such a situation. A gain of function means the gene product does something different, or something additional to its normal function. Even if there are "healthy" other copies of the gene or redundant genes around, the modified function of the introduced allele will interfere with the normal behaviour of the molecular pathways it partakes in, and thus cause a phenotype.

A classic example are constitutively-active alleles. Imagine for example a tumour suppressor with a mutation that prevents it from being degraded (but imagine you don't know this gene is a tumour suppressor yet). Let's say there are other tumour suppressor genes around, so if you knock out your gene of interest, the cell behaves as normal - loss-of-function isn't useful to study what the gene does. Now, if you introduce this constitutively-active mutant variant that doesn't get degraded, it causes too much suppressive activity and drives the cell into complete cell cycle arrest, regardless of the other redundant genes being around. Based on this phenotype, you could infer that the gene is probably a tumour suppressor.

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