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Genetic variability

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Title: Genetic variability  
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Genetic variability

Genetic variability (vary + liable - to or capable of change) is the ability, i.e. capability of a biological system – individual and population – that is changing over time. The base of the genetic variability is genetic variation of different biological systems in space.[1][2][3]

Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another, also. Variability is different from genetic diversity, which is the amount of variation seen in a particular population.[4] The variability of a trait describes how much that trait tends to vary in response to environmental and genetic influences.[4] Genetic variability in a population is important for biodiversity,[5] because without variability, it becomes difficult for a population to adapt to environmental changes and therefore makes it more prone to extinction.

Variability is an important factor in personalized medicine given the rise of the human genome project and efforts to map the extent of human genetic variation such as the HapMap project.

Contents

  • Causes 1
  • Factors that decrease genetic variability 2
  • See also 3
  • References 4

Causes

There are many sources of genetic variability in a population:

  • homologous chromosomes from the male and female parents cross over one another and exchange genetic material. The chromosomes then split apart and are ready to form an offspring. Chromosomal crossover is random and is governed by its own set of genes that code for where crossovers can occur (in cis) and for the mechanism behind the exchange of DNA chunks (in trans). Being controlled by genes means that recombination is also variable in frequency, location, thus it can be selected to increase fitness by nature, because the more recombination the more variability and the more variability the easier it is for the population to handle changes.[6]
However, recombination during meiosis appears to largely reflect homologous recombinational repair of DNA damages that would otherwise be deleterious to the gametes being produced by meiosis.[7] Thus meiotic processes produce recombinational genetic variation as a byproduct of DNA repair and the level of this variation is related to the level of DNA damaging conditions.
  • Immigration, emigration, and translocation – each of these is the movement of an individual into or out of a population. When an individual comes from a previously genetically isolated population into a new one it will increase the genetic variability of the next generation if it reproduces.[8]
  • Polyploidy – having more than two homologous chromosomes allows for even more recombination during meiosis allowing for even more genetic variability in one's offspring.
  • Diffuse centromeres – in centromeres. Being diffused allows the chromatids to split apart in many different ways allowing for chromosome fragmentation and polyploidy creating more variability.[9]
  • Genetic mutations – contribute to the genetic variability within a population and can have positive, negative, or neutral effects on a fitness.[10] This variability can be easily propagated throughout a population by natural selection if the mutation increases the affected individual's fitness and its effects will be minimized/hidden if the mutation is deleterious. However, the smaller a population and its genetic variability are, the more likely the recessive/hidden deleterious mutations will show up causing genetic drift.[10]
DNA damages are very frequent, occurring on average more than 60,000 times a day per cell in humans due to metabolic or hydrolytic processes as summarized in DNA damage (naturally occurring). Most DNA damages are accurately repaired by various DNA repair mechanisms. However, some DNA damages remain and give rise to mutations.
It appears that most spontaneously arising mutations result from error prone replication (trans-lesion synthesis) past a DNA damage in the template strand. For example, in yeast more than 60% of spontaneous single-base pair substitutions and deletions are likely caused by translesion synthesis.[11] Another significant source of mutation is an inaccurate DNA repair process, non-homologous end joining, that is often employed in repair of DNA double-strand breaks.[12] (Also see Mutation.) Thus it seems that DNA damages are the underlying cause of most spontaneous mutations, either because of error-prone replication past damages or error-prone repair of damages.

Factors that decrease genetic variability

There are many sources that decrease genetic variability in a population:

  • genetic drift to lower genetic diversity.
    • An example of a species disrupted by habitat fragmentation is displayed in Queensland koalas over the past century. After seeing their population's size almost divide in half due to human-caused habitat destruction, the koalas lost their ability to migrate between sub-populations. This event resulted in populations with low genetic diversity, especially those more isolated than others.
  • The founder effect is an event that results in populations with low genetic diversity. The founder effect occurs when a population is founded by few individuals, resulting in poor sampling of alleles in a population.
    • An example of a population that experienced the founder effects is displayed with Afognak Island elk population. The population was founded by eight individuals in the early 1900s and quickly grew to a size 1,400. However, even when the population size increased, the genetic variation remained low.
  • Climate change is the drastic change in annual weather patterns. These changes in weather patterns can yield negative consequences for genetic diversity. Driving species out of their fundamental niche, climate changes can lower population size and genetic variation drastically.

See also

References

  1. ^ Rieger R. Michaelis A., Green M. M. (1976): Glossary of genetics and cytogenetics: Classical and molecular. Springer-Verlag, Heidelberg - New York, ISBN 3-540-07668-9; ISBN 0-387-07668-9.
  2. ^ Mayr E. (1970): Populatiomns, species, and evolution – An abridgment of Animal species and evolution. The Belknap Press of Harvard University Press, Cambridge, Massachusetts and London, England, ISBN 0-674-69013-3.
  3. ^ Dobzhansky T. (1970): Genetics of the evolutionary process. Columbia, New York, ISBN 0-231-02837-7.
  4. ^ a b Variation and Variability. Yale University. 1995. Retrieved 2007-05-24. 
  5. ^ Sousa, P., Froufe, E., Harris, D.J., Alves, P.C. & Meijden, A., van der. 2011. Genetic diversity of Maghrebian Hottentotta (Scorpiones: Buthidae) scorpions based on CO1: new insights on the genus phylogeny and distribution. African Invertebrates 52 (1).[1]
  6. ^ Burt, Austin (2000). "Perspective: Sex, Recombination, and the Efficacy of Selection—Was Weismann Right?". Evolution: International Journal of Organic Evolution (54.2): 337–351. 
  7. ^ Harris Bernstein, Carol Bernstein and Richard E. Michod (2011). Meiosis as an Evolutionary Adaptation for DNA Repair. Chapter 19 in DNA Repair. Inna Kruman editor. InTech Open Publisher. DOI: 10.5772/25117 http://www.intechopen.com/books/dna-repair/meiosis-as-an-evolutionary-adaptation-for-dna-repair
  8. ^ Ehrich, Dorothy and Per Erik Jorde (2005). "High Genetic Variability Despite High-Amplitude Population Cycles in Lemmings". Journal of Mammalogy 86 (86.2): 380–385.  
  9. ^ Linhart, Yan and Janet Gehring (2003). "Genetic Variability and its Ecological Implications in the Clonal Plant Carex scopulurum Holm. In Colorado Tundra". Arctic, Antarctic and Alpine Research 35 (35.4): 429–433.  
  10. ^ a b Wills, Christopher (1980). Genetic Variability. NewYork: Oxford University Press. 
  11. ^ Kunz BA, Ramachandran K, Vonarx EJ (April 1998). "DNA sequence analysis of spontaneous mutagenesis in Saccharomyces cerevisiae". Genetics 148 (4): 1491–505.  
  12. ^ Huertas P (January 2010). "DNA resection in eukaryotes: deciding how to fix the break". Nat. Struct. Mol. Biol. 17 (1): 11–6.  
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