Gene duplication also known as chromosomal duplication or gene amplification is any duplication of a region of DNA that contains a gene. The second copy of the gene is often free from mutations of it and have no deleterious effects to its host organism. Thus it accumulates mutations faster than a functional single-copy gene, over generations of organisms. Duplication is the opposite of a deletion. Duplications arise from an event termed unequal crossing-over (a type of gene duplication event that deletes a sequence in one strand and replaces it with duplication) that occurs during meiosis between uneven homologous chromosomes. The chance of this happening is a function of the degree of sharing of repetitive elements between two chromosomes. The product of this recombination is duplication at the site of the exchange and a reciprocal deletion. Gene duplication is considered to be the driving force in the introduction of novel functions.
Gene duplication plays a major role in evolution and genome duplications seems to be a common occurrence. According to Ohno who is a famous author argues in his book of 'Evolution by gene duplication', that evolution by the gene duplication might be one of the most important evolutionary factor. Plants are one of the abundant genome duplicators. The entire yeast genome underwent duplication about 100 million years ago. Wheat is an example of hexaploid (a kind of polyploid), which means it has six copies of its genome. How this helps in the evolutionary process is that when an additional copy id created the copy is free from selective pressure and the new copy of the gene can mutate without deleterious consequence to the organism.
Gene duplication allows for the mutation of new genes that could go both ways, it could potentially increase the fitness of the organism or code for a new function. For example the mutation in ice fish which is the duplicated digestive enzyme resulted in an antifreeze gene. Gene duplication also helps in producing an alternative path or a new path in evolution. If there are multiple gene copies it doesn't seem to add any confusion but indeed both the copies kick in to help in the same function. But tandem repeats in the multicopy genes seems to be relatively unstable. It could have resulted in an increase or decrease in the number. If there are two copies A and B and lets say A is defective then it is most likely that A will be lost and the functional B is retained.
The different mechanisms by which genes become duplicated are often classified on the basis of the size of duplication generated, and whether they involve an RNA intermediate. ‘Retrotransposition’ describes the integration of reverse transcribed mature RNAs at random sites in a genome. A recent research for impulsive duplications in ‘yeast’ suggests that replication-dependent chromosome breakages also take part in a major task in creating tandem duplications (duplication of (single) genes that create tandem repeats in the genome). Genome duplication events generate a duplicate for every gene in the genome, representing a huge opportunity for a step-change in organism’s complexity. Though, genome duplication presents significant problems for the truthful transmission of a genome from one generation to the next, and is as a result a rare event, at least in Metazoa. In principle, genome duplications should be easily identified through the coincident emergence within a phylogeny of many gene families. Regrettably, this signal is knotty by subsequent gradual loss and gain of gene family members. As a result, there is dispute over possible ancient genome duplication events in early vertebrate evolution and more recently in teleost fish, both of which must have occurred hundreds of millions of years ago.
There are many examples of gene duplications, the pancreatic digestive enzyme RNASE 1 in monkeys which breaks down the bacterial RNA and most of the monkeys have only one gene encoding this enzyme but the Asian monkey douc langur has 2 one encodes of RNASE1 and its duplicate which is RNASE1B. While RNASE1 works at a pH of 7.4 and the RNASE1B works at pH of 6.3 and it seems the second one works better at higher acidic conditions. The douc langur has an acidic intestine and the RNASE1B seems to be super efficient at performing this task indeed better than RNASE1 which was the original gene code.and it seems the second one works better at higher acidic conditions. So then why are they duplicate copies if RNASE1B is better, The reason explained is that The RNASE1 has two functions, one is to digest the dietary RNA and the second function is to degrade the double stranded RNA probably protection against viruses. While the RNASe1B is good in digesting the dietary RNA it does not have the ability to degrade the double stranded RNA and so the RNASE1 still carries out this second function. So before the duplication it was one enzyme doing two jobs but after duplication it is two enzymes doing one job but one is better than the other.
Another example of gene duplication can be found in the mouse family, a short lineage of "Mus musculus" has created at least 60 gene duplicates within 1.7 million years; other lineages such as the sibling taxa "Mus caroli", a group that diverged 2.5 million years ago, contain few duplicates.