Population Genetics Activity: A Hardy-Weinberg Equilibrium Example

Introduction: How can you tell if a population that you are studying is evolving? In short, you have to know what happens to a population that is not evolving. A non-evolving population is said to be in equilibrium, also known as Hardy-Weinberg equilibrium. In this activity, you will review a basic population genetics model and then use the Hardy-Weinberg principle to analyze the features that distinguish evolving populations from populations in equilibrium.

Allelic frequencies: p + q = 1, where ‘p’ is the frequency of the dominant allele and ‘q’ is the frequency of the recessive allele.

With this information, genotypic frequencies can be calculated: p2 + 2pq + q2 = 1, where p2 (homozygous dominant, ex: AA), 2pq (heterozygous ex: Aa), and q2 (homozygous recessive Ex: aa).

Hardy and Weinberg argued that if five conditions are met, the population’s allele and genotype frequencies will remain constant from generation to generation. These conditions are as follows:

1)The breeding population is large.

2)Mating is random.

3)There is no mutation of the alleles.

4)No differential migration occurs.

5)There is no selection (All genotypes have an equal chance of surviving).

PTC Activity (phenylthiocarbamide):

Background Notes: A bitter taste reaction to PTC is evidence of the presence of the dominant allele (either AA or Aa). No taste of the chemical represents the lack of the dominant allele (aa).

1) Using the PTC taste-test papers provided, tear off a short strip and press it to your tongue tip. PTC tasters will sense a bitter taste. These are the “tasters.”

2) A decimal number representing the frequency of the tasters (p2 + 2pq) should be calculated by dividing the numbers of tasters in the class by the total number of students in the class. Do the same for the non-tasters (q2). Record your data in the data table.

3) Use the Hardy-Weinberg equation to determine the frequencies (p and q) of the two alleles. The frequency q can be calculated by taking the square root of q2. One q has been determined, you can figure out p using p + q = 1. Record the values in the table and compare p and q to the North American population.

Data Table:

Phenotypes / Allele Frequency Based on the H-W Equation
Tasters (p2 + 2 pq) / Non-Tasters (q2) / p / q
Class Population / # / % / # / %
No. American Population / 0.55 = 55% / 0.45 = 45%

Really bitter or somewhat bitter? ______

p2 = ______2 pq= ______q2 = ______

Analysis: Please answer the questions below using complete sentences.

1) Why do you think there is a difference between our results and the North American population results? Explain.

2) Why would the human population most likely never be in equilibrium, according to the Hardy-Weinberg theory?

3) In Drosophila, the allele for normal length wings is dominant over the allele for vestigial wings (vestigial wings are stubby little curls that cannot be used for flight). In a population of one thousand, 360 show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? (Hint: figure out the q2 % first).

4) The allele for unattached earlobes is dominant over the allele for attached earlobes. In a population of 500 individuals, 25 % show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

5) In certain African countries 4% of the newborn babies have sickle cell anemia, which is a recessive trait. Out of a random population of 1,000 newborn babies, how many would you expect for each of the three possible genotypes?

6) CHALLENGE: In a certain population, the dominant phenotype of a certain trait occurs 91% of the time. What is the frequency of the dominant allele?