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Unformatted text preview: Population Genetics Reading Freeman, Chapter 24 for more info, check out : http://www.zoology.ubc.ca/~whitlock/bio434/LectureNotes/LectureNotes.html an old friend of mine, and a genius, best set of notes on the subject anywhere. also try http://www.utm.edu/~rirwin/391lecHWE.htm Rebecca Irwin’s lectures on PopGen inspired the following lecture, and I have drawn heavily from hersthanks. The Population is the Basic Unit of Evolutionary Change ■ The genotype of an individual is, essentially, fixed at birth. ■ The POPULATION is the smallest unit where evolutionary change is possible. – Unlike individuals, populations permit the origin of new alleles through mutation, and the change in the frequency of alleles through selection, genetic drift, etc.. ■ Individuals do not evolve, populations and species evolve... Population Genetics ■ Population genetics refers to the study of evolution via the observation and modeling of allele frequencies and genetic change in populations of organisms. ■ There are three parameters to keep in mind: – allele frequency: the proportion of a specific allele at a given locus, considering that the population may contain from one to many alleles at that locus. – genotype frequency: the proportion of a specific genotype at a given locus, considering that many different genotypes may be possible. – phenotype frequency: the proportion of individuals in a population that exhibit a given phenotype. The Gene Pool (allele pool) ■ The gene pool (allele pool) consists of all the alleles at all the loci in the population. ■ Consider a population of N organisms. ■ Suppose that they are diploid and reproduce sexually. ■ Consider one gene with two alleles, A and a . ■ The possible genotypes are therefore: ■ AA, Aa, and aa. ■ Phenotype Frequencies ■ To calculate the frequency of a phenotype, count the number of individuals with that phenotype, and divide by the total. Therefore, the frequency of the yellow phenotype in the population below is 4/10=.40 Allele Frequencies ■ Imagine a simple system, with one locus, and two alleles, A and a . – To calculate the frequencies of alleles A and a , count all the alleles in the population, and determine the proportion of them that are A and the proportion of them that are a . – So, if # A means the number of A alleles, and # a means the number of a alleles, and N is the number of individuals in the population, then: ■ Freq (A)=p= (#A)/(2N) Freq(a)=q=(#a)/(2N) – Note that #A + #a = 2N because each individual has two alleles, so the total number of alleles in the population is twice the number of individuals in the population....
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 Spring '08
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 Genetics, Evolution, Population Genetics, aa aa aa

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