Genetic variability

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Introduction

Genetic Variability

The amount by which individuals in a population differ from one another due to their genes, rather than their environment. The study of genetic variability is that of population genetics. Variability is different from variation in that it is the potential to vary rather than the actual variation seen in a population[8]. The rate of the variability of a trait is likely change rather than the actual variations one trait can have.[8] Genetic variability in a population is very important because without it, it becomes difficult for a population to adapt to environmental changes creating a static population.

Causes of Variability

There are many sources of genetic variability in a population:

• Genetic Recombination is one source of variability; during meiosis in sexual creatures two homologous chromosomes from the male and the female cross over one another and exchange gene sequences. The chromosomes then split apart and are ready to form an offspring. The cross-over 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[5].

• 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[4].

• 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[2].

• Genetic Mutations- mutations are accidental mistakes made in transcription, translation, and all the other processes that DNA and RNA are put through before the creation of a protein. They can be positive, negative, or neutral in regards to fitness, and because they are random they contribute to the genetic variability within a population [7]. This variability can be easily propagated throughout a population by natural selection if the mutation is beneficial to fitness and its affects will be minimized/hidden if the mutation is found to be deleterious to individual fitness. However, the smaller a population and its genetic variability are, the more likely the recessive/hidden deleterious mutations will show up causing genetic drift[7].

Measuring Variability

There are many ways to measure the genetic variability both within a population, between populations, and between different but closely related species.

a) An older method for measuring variability is allozyme or isozyme electrophoresis. Isozymes are enzymes that differ in their sequence of amino acids but still can catalyze the same reactions, while allozymes are enzymes from different alleles on the same gene [12]. Both allozymes and isozymes are highly variable and can be studied using gel electrophoresis which separates proteins and enzymes by speed and charge and displays them in bands using stains [12]. Once a gel is made you can compare protein sequences between individuals and populations to see which allozymes or enzymes are polymorphic and which are monomorphic in your population or species[3]. The number of polymorphisms per tested allozymes or enzymes can be a useful percentage in describing genetic variability in a population.

b) A second method for measuring genetic variability is microsatelite analysis. Microsatellites are a unit of simple nucleotide sequence repetitions usually found in non-coding regions of the genome. The number of times a nucleotide sequence repeats in a microsatellite can vary (AGCTAGCT vs. AGCTAGCTAGCTAGCT) giving microsatellites of different lengths amongst individuals in a population. This variation in microsatellite length can easily be measured by amplifying the region with PCR and then sending it through electrophoresis gel to show bands. There are usually two length s a microsatellite unit can be, so if both chromosomes in an individuals DNA contain one type then they are homozygous and if the animals chromosome has both types then they are considered heterozygous at that site[11].

c) Another way to measure variability is by sequencing the Mitochondrial DNA’s control region which is passed only by the mother to her offspring without any sort of recombination so it makes it easy to track matrilineage in a population using it. Also because there is no recombination, mtDNA has a high rate of mutations making it useful in measuring variability. MtDNA is measured by sequencing DNA using PCR and electrophoresis [3].

Uses

1. Restoration: In restorations one must must know the variability of the plants you’re placing into a site, so that one does not don’t accidentally inbreed the population causing genetic erosion[1]
2. Reserve design- here the goal is to use reserve design to maximize genetic variability so that the population will be able to adapt to new environmatal problems[2].
3. Can use genetic variablity to learn about the reproductive strategies of certain organisms, such as whether a plant species is asexual or sexual the majority of the time[2].
4. Can use measurements of gentic variability to learn the frequency of immigration and between which populations a species exchanges genes[3]
5. Can use measurements of genetic variability to learn how a species recovers from low population densities after a crash[4]
6. Agriculture- genetic variability is important in agriculture because if it is poor a population has lower resistance to disease and pests, which may lead to blights when environmantal fastors change[1]
7. Evolution- uses genetic variability to understand why certain life strategies may have been chosen, such as asexuality in plants []
8. Can use to find what places a local population migrated from in the past[11][6].
9. Species reintroduction- can use genetic variability to decide which species most likely originally lived in the location you want to repopulate, thus making it most fit to be reintroduced there[11].

References

1. Rogers, Deborah, L. “Genetic Erosion: No longer just an agricultural issue.” Native Plants Journal 5.2 (2004): 112-122. http://muse.jhu.edu/journals/native_plants_journal/v005/5.2rogers.html
2. Linhart, Yan and Janet Gehring. “Genetic Variability and its Ecological Implications in the Clonal Plant Carex scopulurum Holm. In Colorado Tundra.” Arctic, Antarctic and Alpine Research 35.4(2003): 429-433.
3. Wilson, Gregory et al. “Genetic Variability of Wolverines (Gulo gulo) from the Northwest Territories, Canada: Conservation Implications.” Journal of Mammalogy 81.1 (2000): 186-196.
4. Ehrich, Dorothy and Per Erik Jorde. “High Genetic Variability Despite High-Amplitude Population Cycles in Lemmings.” Journal of Mammalogy 86.2: (2005): 380-385.
5. Burt, Austin. “Perspective: Sex, Recombination, and the Efficacy of Selection—Was Weismann Right?” Evolution: International Journal of Organic Evolution 54.2 (2000) : 337-351.
6. Zaldivar, Maria et al. “Genetic Variation of Mantled Howler Monkeys (Alouatta palliate) from Costa Rica.” Biotropica 35.3 (2003): 375-381.
7. Wills, Christopher. Genetic Variability. NewYork: Oxford University Press, 1980.
8. Variation and Variability. 1995. Yale University. 24 May 2007. http://www.cbc.yale.edu/old/cce/papers/ALife/node2.html
9. Gene Primer. UVM Education Center. 24 May 2007. http://www.uvm.edu/~cgep/Education/Microsatellite.html
10. Allozyme Electrophoresis and Population Structure in the Snowy Campion. Vanderbilt University. 24 may 2007. http://www.cas.vanderbilt.edu/bsci111b/allozymes/supplemental.htm
11. Wayne, Robert, K and Phillip A. Morin. "Conservatin Genetics in the New Molecular Age." Frontiers in Ecology and the Environment 2.2 (2004): 89-97.