Gene redundancy

Gene redundancy is the existence of multiple genes in the genome of an organism that perform the same function. This is the case for many sets of paralogous genes. When an individual gene in such a set is disrupted by mutation or targeted knockout, there can be little effect on phenotype as a result of gene redundancy, whereas the effect is large for the knockout of a gene with only one copy.

Gene redundancy most often results from Gene duplication. When a gene is duplicated within a genome, the two copies are initially functionally redundant. Three of the more common mechanisms of gene duplication are retroposition, unequal crossing over, and non-homologous segmental duplication. Retroposition is when the mRNA transcript of a gene is reverse transcribed back into DNA and inserted into the genome at a different location. During unequal crossing over, homologous chromosomes exchange uneven portions of their DNA. This can lead to the transfer of one chromosome's gene to the other chromosome, leaving two of the same gene on one chromosome, and no copies of the gene on the other chromosome. Non-homologous duplications result from replication errors that shift the gene of interest into a new position. A tandem duplication then occurs, creating a chromosome with two copies of the same gene. Figure 1 to the left provides a good visual explanation of these three mechanisms.

As the genome is replicated over many generations, the redundant gene's function will most likely evolve due to Genetic drift. This means that the gene will acquire mutations that change the overall function. The three mechanisms of functional differentiation in genes are nonfunctionalization (or gene loss), neofunctionalization and subfunctionalization.

During nonfunctionalization, or degeneration/gene loss, one copy of the duplicated gene acquires mutations that render it inactive or silent. At this time, the gene has no function and is called a pseudogene. Pseudogenes can be lost over time due to genetic mutations. Neofunctionalization occurs when one copy of the gene accumulates mutations that give the gene a new, beneficial function that is different than the original function. Subfunctionalization occurs when both copies of the redundant gene acquire mutations. Each copy becomes only partially active; two of these partial copies then act as one normal copy of the original gene. Figure 2 to the right provides a visual explanation.

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