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BGY Chapter_6.doc

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University of Toronto Mississauga

Chapter 6 (Pg. 169 – 188 & 194 – 207) Mendelian Genetics in Populations I: Selection and Mutation as Mechanisms of Evolution 6.1. Mendelian Genetics in Populations: The Hardy-Weinberg Equilibrium Principle • Population genetics begins with a model of what happens to allele and genotype frequencies in an idealized population. Once we know how Mendelian genes behave in the idealized population, we will be able to explore how they behave in real populations. o A population is a group of interbreeding individuals and their offspring o The crucial events in the life cycle of a population are:  The adults produce gametes  The gametes combine to make zygotes  The zygotes develop into juveniles  Juveniles grow up to become the next generation of adults Simulation This chapter will be simulating a population of mice to explain the material • All the eggs and sperm produced by all the adults in the population are dumped together in a barrel and stirred. o This barrel is known as the gene pool. • Imagine that 60% if the eggs and sperm received a copy of allele A and 40% received allele a. o This means the frequency of allele A is 0.6 and of a is 0.4. • Using a simulation, 34 mice had genotype AA, 57 had Aa and 9 had aa. o Assuming that each mouse donates 10 gametes to the gene pool:  The 34 AA adults together make a total of 340 games: 340 carry allele A and none carry allele a.  The 57 Aa adults together make a total of 570 gametes: 285 carry allele A and 285 carry allele a.  The 9 aa adults together make a total of 90 gametes: none carry allele A and 90 carry allele a. o Thus 625 in total carry allele A and 375 carry allele a, for a total of 1000 gametes. The frequency of gametes in the new gene pool is 0.625 for allele A and 0.375 for allele a. • In simulated populations allele frequencies change somewhat across generations. This is evolution resulting from blind luck. o Blind luck causing populations to evolve unpredictably is an important result of population genetics. o This mechanism of evolution is called genetic drift. Numerical Calculation Read pages 174 – 176 for visual reference. • Numerical examples show that when blind luck plays no role, allele frequencies remain constant from one generation to the next. The General Case Read pages 177 – 179 for visual reference. • The math on these pages prove that any allele frequency can remain constant and at equilibrium for numerous generations without external interference • This is known as the Hardy-Weinberg Equilibrium Principle. It is based on two conclusions: o The allele frequencies in a population will not change, generation after generation o If the allele frequencies in a population are given by p and q, the genotype frequencies will be given by p2, 2pq and q 2 What Use Is the Hardy-Weinberg Equilibrium Principle? What makes it useful is that it rests on a specific set of simple assumptions. When one or more of these assumptions is violated, the Hardy-Weinberg conclusions no longer hold. • There is no selection o All members of the model pop. survived at equal rates and contributed equal number of gametes to the gene pool. When this assumption is violated (some survive better than others), the frequencies of alleles may change from one generation to the next. • There is no mutation o In the model population, no copies of existing alleles were converted by mutation into copies of other existing alleles, and no new alleles were created. When this assumption is violated, allele frequencies may change from one generation to the next • There is no migration o No individuals moved into or out of the model population. When this assumption is violated, individuals carrying some alleles move into or out of the population at higher rates than individuals carrying other alleles. • There are no chance events o Blind luck plays no role o When this assumption is violated, and by chance some individuals contribute more alleles to the next generation than others, allele frequencies may change from one generation to the next. This is known as genetic drift. • Individuals choose their mates at random o Unlike the first four assumptions, if this assumption is violated—species choose to mate those of the same genotype—allele frequencies do not change from one generation to the next but rather genotype frequencies do. o Shifts in combination w/ violation of the other assumptions lead to evolutionary change. When any of these five assumptions are violated, it is an indication that the population is heading towards evolution. Changes in the Frequency of the CCR5-∆32 Allele Read page 182. 6.2. Selection • Selection happens when individuals with particular phenotypes survive to reproductive age at higher rates than individuals with other phenotypes, or when individuals with particular phenotypes produce more offspring during reproduction than individuals with other phen
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