Calculation of Allele and Genotype Frequencies

Calculation of Allele and Genotype Frequencies -...

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INTRODUCTION Population geneticists study frequencies of genotypes and alleles within populations rather than the ratios of phenotypes that Mendelian geneticists use. By comparing these frequencies with those predicted by null models that assume no evolutionary mechanisms are acting within populations, they draw conclusions regarding the evolutionary forces in operation. In a constant environment, genes will continue to sort similarly for generations upon generations. The observation of this constancy led two researchers, G. Hardy and W. Weinberg, to express an important relationship in evolution. The law that describes this relationship bears their names. The Hardy-Weinberg Equilibrium Theory serves as the basic null model for population genetics. Every individual has alleles that were passed on from their parents. If we take all of the alleles of a group of individuals of the same species (that is, a population) we have what is called the gene pool. The frequency, or proportion, of individuals in that population that possess a certain allele is called the allele frequency. Populations can have allele frequencies, but individuals cannot. This obviously makes populations the best hierarchical unit, or level, to study evolution, as evolution is basically the study of the change in allele frequencies over time. Allele Frequencies Consider an individual locus and a population of diploid individuals where two different alleles, A and a, can be found at that locus. If your population consists of 100 individuals, then that group possesses 200 alleles for this locus (100 individuals x 2 alleles at that locus per individual). The number of A alleles present in that population expressed as a fraction of all the alleles (A or a) at that locus represents the frequency of the A allele in the population. 1. To calculate allele frequencies for populations of diploid organisms, first multiply the number of individuals in the population by 2 to obtain the total number of alleles at that locus. 2. Select one of the alleles for your first set of calculations. Let’s first choose the A allele from the example provided above. a. Individuals homozygous for the A allele will each possess 2 A alleles. Multiply the number of AA homozygotes by 2 to calculate the number of A alleles. b. Heterozygotes will each possess only one A allele.
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This essay was uploaded on 03/27/2008 for the course BIO 014 taught by Professor Chew during the Spring '08 term at Tufts.

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Calculation of Allele and Genotype Frequencies -...

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