Navigation Links
Muller's ratchet


In genetics, Muller's ratchet is a hypothesis that explains how functionally important genes may be lost when organism's genes are only transmitted vertically, without recombination caused by sex. Exclusive vertical transfer occurs when the organism is an endosymbiont, e.g. a bacterial endosymbiont of insects, which is only transmitted to offspring from the mother. Another example is mitochondria and chloroplasts, which are also transmitted only vertically (although, in this case, they are not generally considered a separate organism).

Muller's ratchet applies to any deleterious mutation that occurs in a vertically transmitted organism. It may be that the mutation is deleterious, but not lethal. Furthermore, the organism with the mutation may have another advantageous mutation -- or its host may have an advantageous mutation -- that will lead to their survival despite the deleterious mutation. Because there is no possibility for genetic mixing, e.g. through sexual reproduction, the organism's descendants have no opportunity to receive a good copy of the gene. If the other mutation is advantageous, their survival means that the deleterious mutation persists. These deleterious mutations resemble the operation of a ratchet, in that the organism can never go back.

By contrast, sexual reproduction allows most plants and animals to create offspring with good copies of two genes via crossover. That is, if one animal has the deleterious mutation in one gene and the advantageous mutation in another, while another animal has two normal copies of the gene, a mating of these animals can produce offspring with the advantageous mutation and without the deleterious one. Thus, a deleterious mutation coupled with an advantageous one can be undone in organisms with sexual reproduction. Horizontal gene transfer in bacteria allows a similar situation. Generally, only obligate endosymbionts, mitochondria and chloroplasts are exposed to Muller's ratchet. But even sexually reproducing organisms are exposed to Muller's ratchet in the case of single-copy chromosomes, such as the human Y chromosome.

Muller's ratchet is an important theory as to why sex evolved, but works better in small populations than larger ones. See the Red Queen for the hypothesis that is generally regarded as being superior, though note that the two are not mutually exclusive. Any discussion on the evolution of sex should include a discussion of the Muller's ratchet.

References

  • Muller, H.J. (1932). "Some Genetic Aspects of Sex". American Naturalist 66:118-138 (Muller's original paper)
  • Nancy A. Moran (1996), Accelerated evolution and Muller’s ratchet in endosymbiotic bacteria, Proceedings of the National Academy of Sciences USA, 93, pp. 2873-2878. [1] (An article that discusses Muller's ratchet in the context of endosymbiotic bacteria.)

'"/>


(Date:3/14/2016)... NXTD ) ("NXT-ID" or the "Company"), a company ... of a new series of commercials on Time Warner Cable ... .  The commercials will air on Bloomberg TV, Fox Business ... show. --> NXTD ) ("NXT-ID" or the "Company"), ... the airing of a new series of commercials on Time ...
(Date:3/10/2016)... India , March 10, 2016 ... a new market research report "Identity and Access Management ... & Audit, Compliance, and Governance), by Organization Size, by ... to 2020", published by MarketsandMarkets, The market is estimated ... USD 12.78 Billion by 2020, at a Compound Annual ...
(Date:3/9/2016)... March 9, 2016  Crossmatch ® , a ... solutions, today announced the addition of smart features ... multi-factor authentication platform. New contextual and application-specific ... step-up security where it,s needed most — while ... DC . --> Washington, ...
Breaking Biology News(10 mins):
Other biology definition