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What is population genetics and how is it put to practical use?

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What is population genetics and how is it put to practical use? All evolutionary changes start with changes within populations (Li 1998). Therefore to gain an understanding about the principles governing evolution we must look at how populations can be subject to evolutionary forces. These forces act upon the genetic variation within a population, and population genetics deals with how evolution drives changes in the genetic structure and variation of populations. However the study of changes in the genetic variation of populations is not unique to evolutionary theory, it is very valuable in disease screening, artificial selection and forensic science to name a few. In this essay I shall describe how population genetics is investigated, from building a null hypothesis model and the reasons for deviation, to the methodology used to put theory into practise and where it is used. Allele Frequency Model Genetic variation is attributable to the existence of different forms of genes (alleles) in populations (polymorphisms). The frequencies of these genes are what contribute to the overall genetic variation, the factor population geneticists are most interested in. The frequencies of genes can be easily modelled and deduced based on the number of individuals in the population that carry them. For example in a diploid population N with two alleles A and a, the allele frequencies can be calculated by working out the distribution of the different genotypes in the population, AA Aa and aa. Dividing the number of individuals in this population that express this genotype by N gives you the frequencies of the genotypes in the population. To work out the proportion of the alleles in the population it is simply: fhomozygous + 1/2 fheterozygous = p (or q for other allele). Note that since there are only two Alleles their proportions must add up to 1 since you cannot encounter any other allele. p+q=1. Genotype Frequency Model This is a very simple model explaining how genotype frequencies are calculated, but once the frequencies are set up how are the genotypes (esp. ...read more.


The converse is also true, deleterious alleles will be removed from the population as they are unable to contribute as much to the next generation as they have reduced fitness. The process of natural selection therefore differs to genetic drifts' random contribution to the next generation in being a selective contribution to the next generation causing a directional change in allele frequencies. Natural selection however does not usually act at the level of individual loci (specific disease resistance may be an exception), rather the entire phenotype is put under selection pressures so some advantageous alleles may never alter gene frequencies because they have other deleterious alleles present of are lost through drift. Overall phenotype fitness is a more important factor than individual allele fitness. The types of shift produced by natural selection in response to the environment allow for even greater variation in the distribution and frequency of genes in a population. The following is a description of the different type of selection, both on the individual loci and how it manifests itself in phenotypic selection. Type of selection and Pictorial Representation Description Purifying or Negative Selection This occurs as in most cases where mutations are deleterious and are actively selected against removing them from the population. This creates the bell shape on the diagram. This is classic equilibrium of phenotypes. Positive or Directional Selection Very rarely a mutant allele arises that confers and advantage to the host and so will be actively selected for, driving the phenotypic distribution in a direction of increased fitness and altering the allele frequency accordingly. An example of this would be the selective sweep of antibiotic resistance in E. coli. Fixation will occur faster than 4N generations. Overdominance or Stabilising selection In some cases the heterozygote, or most numerous phenotype in the population, is favoured and selected for, this applies to both the loci (Overdominance) and the phenotype (stabilising selection). ...read more.


Using population genetics in recent years has become increasingly important in the medical field. Genetic diseases can be identified through the techniques used to study individual genetic variation and it is possible to find carriers and take appropriate steps to reduce the incidence of the disease. Also the modelling behind the genetic variation within a population allows governments to assess what screening procedures it needs to perform as an offset to the cost of treating individuals and the overall cost to the fitness of the population. An interesting example is found concerning the incidence of Taysachs. Taysachs is an autosomal recessive disease and can be identified by looking at the activity of Hexosominidase A. The local government wanted to see if it was feasible to screen the population for carriers to reduce the costs of treatment on the health service. The cost would have been $2x107 to screen the population and the chances of carriers mating is one in 107 and even then the chances of producing a homozygote were 1 in 4. Simple economics ruled out the possibility of a population wide screen for carriers. However using population genetics a small sub population, the Ashkenazi Jews, were identified as having an abnormally high incidence of the disease 1 in 30. By screening only the females in that population, and then male partner if a carrier identified, it turned out to be economically feasible to screen this population and within a single generation the incidence of the disease was reduced 95%. (D Roberts 2001 Lecture). Although this is only a brief summary of what population genetics can do for us, it illustrates that its usefulness extends beyond that of proving hypothesis and can be used to have a positive impact on people's lives. Population genetics will become increasingly important over the next few decades as models and methodology is refined and the applications for this tool will become increasingly widespread. ...read more.

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