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Fixation of Dominant Alleles Start with a population that has a gene with two alleles A and a with classical Mendelian dominance that are at equal frequency

  1. Fixation of Dominant Alleles
  • Start with a population that has a gene with two alleles (A and a) with classical Mendelian dominance that are at equal frequency (p = 05 q = 05) Assume this first generation is at hardy Weinberg equilibrium
  • Calculate the genotype frequencies: AA =

Aa =

aa =

  • Now assume some environmental change that makes the recessive phenotype completely unfit (fitness = 0) Calculate the allele frequencies and genotype frequencies in the second generation (Hint: Your calculations might be easier if you assume a population size of 100)

p = AA =

q = Aa =

aa =

What was the reduction in the frequency of the recessive allele from generation #1 to generation #2?

  • Assuming the same fitness for the recessive phenotype, calculate the allele frequencies and genotype frequencies in the third generation
  • p = AA =
  • q = Aa =
  • aa =
  • What was the reduction in the frequency of the recessive allele from generation #2 to generation #3?
  • There a difference in the change in allele frequencies between generations even though the level of selection remained the same Why?
  • What type of selection is this and what would happen if we continued this process over many generations?
  1. Open a web browser and go to http://wwwradfordedu/~rsheehy/Gen_flash/popgen/

This website provides a simulation of allele frequency changes due to genetic drift and natural selection for a two allele system The program needs to have Flash installed, if on a campus computer you should be able to access a working version using Google Chrome

  • Perform simulations for five populations using the default settings for allele frequency and population size (p = 05, N = 50) and the number of Generations set to 200
  • How many of the simulations fixed the p allele at 1?
  • How many of the simulations lost the p allele completely?
  • Repeat 1a Were your results the same? Why or why not?
  • Now change the population size to 1000 () and run five simulations
  • Were your results significantly different than in 1a and 1b? (Hint: What’s the difference between short term allele frequency fluctuations?)
  • The graph at right represents the frequency of a neutral allele (A1) in a population as it becomes fixed
  • If the allele is neutral, what effect will natural selection have on it?
  • Would you predict that this allele will become fixed or be lost?

Did the population size change during the time period covered by the graph? How do you know?

May 01 2020 View more View Less

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