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H-W%20equil - HARDY-WEINBERG EQUILIBRIUM At the time that...

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HARDY-WEINBERG EQUILIBRIUM At the time that Mendel's work was rediscovered, people began to question if "dominant genes" (alleles) shouldn't "take over" and spread through the population. Hardy and Weinberg both published rational explanations of why gene frequencies will not change "unless" forced to do so. Basically, if there is "random mating" with regard to a trait (that is, matings are made without consideration of the trait), the frequencies of the dominant and recessive alleles in the population will also be the frequencies found in the gametes. Thus when there is random mating, the genotypic frequencies in the next generation will be p 2 (AA) : 2pq (Aa) : q 2 (aa), and the allele frequencies will still be p and q. This situation is referred to as Hardy- Weinberg Equilibrium The " forces " that can change gene frequencies are; • " Drift " or chance fluctuations Mutation Migration Selection p (A) q(a) p (A) q(a) eggs sperm p 2 (AA) pq(Aa) pq(Aa) q 2 (aa) or p 2 AA + 2pq Aa + q 2 aa
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Drift will be the primary factor affecting gene frequency when populations are small. If the reproductive population only contains a few individuals it is not surprising that chance is a major factor. For example if we closed our eyes and counted out 10 jelly beans from a bowl that contained an even mix of white and black beans, we would not be surprised if we ended up with more of one color than the other, or if by chance we got 7 white and 3 black beans. In genetics, to get to the next generation, we would next draw from a bowl that had 70% white and 30 % black beans, rather than the 50:50 split we started with. Then it would not be surprising if we happened to get 6:4 or 8:2 in the next draw. If we follow the same procedure over several generations, we will end up at " fixation" ie, where all the (alleles) in a sample are either white or black. From then on, we will be drawing from populations where only one type of allele is present.
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