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Gene duplication occurs when an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome leads to the duplication of a region of DNA containing a gene . The significance of this process for evolutionary biology is that, unlike a single functional gene, which is usually subject to purifying selection and thus has a slowed mutation rate, one copy of a duplicate set of genes is often freed from selective pressure, allowing it to freely mutate. This is because with two copies of a gene present, mutations in just one copy of the gene often have no deleterious effect on the organism; thus, the second copy is free to "explore" the sequence space by mutating randomly. The duplicate gene may either (a) acquire mutations that lead to a gene with a novel function or (b) acquire deleterious mutations and become a pseudogene.
The postulate that gene duplication has a major role in evolution was developed by Susumu Ohno in his classical book “Evolution by gene duplication” (1970) and is now widely accepted as a major evolutionary force. Some have argued that gene duplication is the most important evolutionary force since the emergence of the universal common ancestor .
Major genome duplication events are not uncommon. It is retrodicted that the entire yeast genome underwent duplication about 100 million years ago. Plants are the most prolific genome duplicators. Wheat, for example, is hexaploid (a specific term for a polyploid organism) meaning it has six duplicate copies of its genome.
The two genes that exists after a gene duplication event are called paralogs. Paralogs usually code for proteins with a different function and/or structure, as stated above. This is different from orthologous genes, which code for proteins with similar functions but exist in different species. Orthologs are created from a speciation event. (See Homology of sequences in genetics).
It is important (but often hard) to differentiate between paralogs and orthologs in biological research. Experiments on human gene function can often be carried out on other species if a homolog to a human gene can be found in the genome of that species, but only if the homolog is orthologous. If they are paralogs and resulted from a gene duplication event, their functions are probably too different.
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