Lesson 2 — Hardy-Weinberg Calculations


  Hardy Weinberg Goldfish Lab

Introduction
This lab allows you to simulate natural selection. Two types of goldfish cookies represent two phenotypes of goldfish, one of which is naturally selected and the other is eaten by a predator. You will record the changes in phenotype frequency over several generations, which will allow you to calculate the changes in allele frequency.
 
Problem
How does natural selection change allele frequencies?
 
Materials
  • large bowl
  • 1 package of pretzel goldfish (brown fish)
  • 1 package of cheese goldfish (gold fish)*
  • paper plates or towels to put fish on
*Note: You may choose to substitute goldfish with other snacks: teddy graham bears (in four flavors), jellybeans, or M & M's etc.
 
Background information
  • Birth rate equals death rate.

  • Mating is random.

  • No mutations occur.

  • Only two alleles occur in the population. Brown (B ) colour is the dominant trait and gold (b ) is the recessive trait.

  • The population should be large.
 
© Getty Images

Procedure
  1. Empty both packages of pretzel and cheese goldfish into a large bowl. Mix well.

  2. Select randomly 10 fish from the bowl. This is your first generation.

  3. Record the number of gold fish (bb) and brown fish (BB and Bb) in the observation chart.

  4. Eat 3 gold fish from the first generation. This is selective predation.

  5. Add 3 random fish from the bowl to the population. This is your second generation.

  6. In the observation table, record the number of gold fish (bb) and brown fish (BB and Bb).

  7. Repeat steps 4 to 6 until you have 5 generations of fish population recorded in your observation chart.

Observation

Generation  Number of Gold Fish
(q2)
 Number of Brown Fish
(p2 and 2pq)
 q2
Frequency of Homozygous Recessive
 q
Frequency of Gold Gene
 p2
Frequency of Homozygous Dominant
 2pq
Frequency of Heterozygous
 1            
 2            
 3            
 4            
 5            


Analysis
Calculate the allele and genotype frequencies for each population.


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Conclusion
What happened to the population of the homozygous recessive gold fish from generation to generation? The Hardy-Weinberg equations help us determine if changes are occurring in the genotype frequencies of a given population. By applying the Hardy-Weinberg equations, we can observe that the frequency of recessive allele is maintained in the gene pool if the five conditions of Hardy-Weinberg equilibrium are met.

Selective predation of gold fish resulted in the reduction of recessive alleles in the subsequent generations. However, note that the recessive allele did not disappear completely from the population. Remember that heterozygous brown fish carry recessive gold genes.

What would happen if selective predation of brown fish occurred in this population? The brown fish population would decrease and the gold fish population would increase. Similarly, if non-random mating occurred, the favoured allele frequency would increase.

This simple application shows that predators can determine the course of natural selection. Humans (outside of natural populations) have no true predators. What replaces the role of predation in natural selection of human populations?

Biology 30 © 2008  Alberta Education & its Collaborative Partners ~ Updated by ADLC 2019