- 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 = 0.5. q = 0.5). 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?
- Open a web browser and go to http://www.radford.edu/~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 = 0.5, 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?
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