Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another. Variability is different from genetic diversity, which is the amount of variation seen in a particular population. The variability of a trait describes how much that trait tends to vary in response to environmental and genetic influences. Genetic variability in a population is important for biodiversity, 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 evolution as it affects an individual's response to environmental stress and thus can lead to differential survival of organisms within a population due to natural selection of the most fit variants. Genetic variability also underlies the differential susceptibility of organisms to diseases and sensitivity to toxins or drugs — a fact that has driven increased interest 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.
There are many sources of genetic variability in a population:
- Homologous recombination is a significant source of variability. During meiosis in sexual organisms, two 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.
- 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. 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.
- 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 asexual organisms where the offspring is an exact genetic copy of the parent, there are limited sources of genetic variability. One thing that increased variability, however, is having diffused instead of localized centromeres. Being diffused allows the chromatids to split apart in many different ways allowing for chromosome fragmentation and polyploidy creating more variability.
- Genetic mutations – contribute to the genetic variability within a population and can have positive, negative, or neutral effects on a fitness. 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.
- 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. 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. (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.
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