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Gene duplication is believed to play a major role in evolution; this stance has been held by members of the scientific community for over 100 years. Susumu Ohno was one of the most famous developers of this theory in his classic book Evolution by gene duplication (1970). Ohno 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 believed that the entire yeast genome underwent duplication about 100 million years ago. Plants are the most prolific genome duplicators. For example, wheat is hexaploid (a kind of polyploid), meaning that it has six copies of its genome.
The duplication of a gene results in an additional copy that is free from selective pressure. One kind of view is that this allows the new copy of the gene to mutate without deleterious consequence to the organism. This freedom from consequences allows for the mutation of novel genes that could potentially increase the fitness of the organism or code for a new function. An example of this is the apparent mutation of a duplicated digestive gene in a family of ice fish into an antifreeze gene.
Another view is that both copies are equally free to accumulate degenerative mutations, so long as any defects are complemented by the other copy. This leads to a neutral "subfunctionalization" or DDC (duplication-degeneration-complementation) model, in which the functionality of the original gene is distributed among the two copies.
The two genes that exist after a gene duplication event are called paralogs and usually code for proteins with a similar function and/or structure. By contrast, orthologous genes are ones which code for proteins with similar functions but exist in different species, and are created from a speciation event.
It is important (but often difficult) 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 likely to be too different.
The paralogous segments can be repeat sequences with more than 90% sequence similarity. In such cases, they are known as low copy repeats (LCRs) though they are not highly repetitive sequences. They are mostly found in pericentronomic, subtelomeric and interstitial regions of a chromosome. The LCRs, due to their size (>1Kb), similarity, and orientation, are highly susceptible to duplications and deletions. These genomic rearrangements are caused by the mechanism of non-allelic homologous recombination. The resulting genomic variation leads to gene dosage dependent neurological disorders such as Rett-like syndrome and Pelizaeus-Merzbacher disease.
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