Genetic Drift Exercise

readings - introduction - exercise instructions - homework - instructor hints

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Readings

Haldane, J.B.S. 1964. A defense of beanbag genetics. Perspectives in Biology and Medicine 7: 343-359.

Mayr, E. 1959. Where Are We? Cold Spring Harbor Symp. Quant. Biol . 24. pp1-14.

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Introduction for students

This exercise illustrates the phenomenon of sampling error, which is the mechanism whereby genetic drift changes allele frequencies in populations. In this example a haploid model is used for simplicity. This exercise is designed to accompany an in-class discussion of genetic drift. 

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Exercise Instructions

• Start by finding a partner and counting out 20 white beans and 20 speckled beans into a lunch bag. For consistency, abbreviate the bean types as follows:
• White Beans: W
• Speckled Beans: S 
• You will be assigned one of the following :
• Randomly draw 4 beans from the initial population (this is your breeding population) in the bag. These four beans are the only ones that reproduce from the initial population. 
• Each of these beans has 10 offspring.
• Create the first generation from the offspring.
• The population size should be 40.
• Calculate the frequencies of the W and S alleles in the first generation.
• Repeat and monitor the changes in gene frequencies over ten generations.
• Repeat the same procedure as in part one with the population size = 40, breeding population size = 10, and initial genotype frequencies = 0.5. In this case, each parent will produce 4 offspring so that the population size remains at 40 from the generation to generation.

Homework

• Compile separate tables for the two different breeding population sizes on the blackboard.
• List the frequencies of W alleles at each generation for all replicate populations in your section.
• Prepare one graph using the class data. For each of the two different breeding population sizes, plot the frequency of the W allele on the x-axis and the number of generations on the y-axis.

Instructor hints