Population Genetics

Population Genetics - Population Genetics Reading Freeman,...

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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 Irwins lectures on PopGen inspired the following lecture, and I have drawn heavily from hers-thanks. 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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Population Genetics - Population Genetics Reading Freeman,...

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