Title: Assignment: Lab - Dragons and HW problems
Course: BIO-182 RTZ

This in-class assignment provides additional practice understanding concepts behind the Hardy-Weinberg equations.  It is worth 5 points towards your lab score.  The handout is included below for your convenience.

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POPULATION GENETICS

BACKGROUND INFORMATION:

Hardy-Weinberg Principle essentially states that if no evolution is occurring, then an equilibrium of allele frequencies will remain in effect in each succeeding generation of sexually reproducing individuals. In order for equilibrium to remain in effect (i.e. that no evolution is occurring) then the following five conditions must be met:

1. No mutations must occur so that new alleles do not enter the population.
2. No gene flow can occur (i.e. no migration of individuals into, or out of, the population).
3. Random mating must occur (i.e. individuals must pair by chance)
4. The population must be large so that no genetic drift (random chance) can cause the allele frequencies to change.
5. No selection can occur so that certain alleles are not selected for, or against.

In effect, the Hardy-Weinberg formulas allow scientists to determine whether evolution has occurred. Any changes in the gene frequencies in the population over time can be detected. In reality, this condition cannot exist in real life. Some or all of the forces mentioned above act on extant populations at various times, therefore populations are subject to evolutionary forces. The Hardy-Weinberg formulas allow us to detect allele frequencies that change from generation to generation, thus allowing a simplified method of determining whether evolution is occurring.

Two formulas provide the foundation for determining changes in allelic frequencies within a population:

    - p^2 + 2pq + q^2 = 1
    - p + q = 1

Use these two formulas to solve problems regarding population genetics.

What the symbols mean:

            - p  = frequency of the dominant allele in the population
            - q  = frequency of the recessive allele in the population
            - p^2 = percentage of homozygous dominant individuals
            - q^2 = percentage of homozygous recessive individuals
            - 2pq = percentage of heterozygous individuals

ACTIVITIES:

A. Solve the following problems:

#1.   -  View the Dragons (see handout)  The winged trait is dominant. Fill in the blanks in the box below.

#2.  Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Given this simple information, calculate the following:

a. The percentage of butterflies in the population that are heterozygous.

b. The frequency of homozygous dominant individuals.

. Real world application:

In this exercise, you will use the class as a sample population to estimate the estimate the frequencies of alleles of a gene controlling the ability to taste the chemical phenylthiocarbamide (PTC).  The ability to taste PTC indicates the presence of a dominant allele, in either the homozygous condition (frequency = p^2) or in the heterozygous condition (frequency = 2pq). The inability to taste the chemical at all depends on the presence of a homozygous recessive genotype (frequency = q^2).  Note: To find the frequency of the PTC-tasting allele, you must find ‘q’.

Objectives: Estimate allelic frequencies for a specific trait within a sample population and compare these with estimated frequencies for the entire population.

Procedure:

1. Tear off a short strip of PTC paper and press it to the tip of your tongue. Record your ability to either taste or not taste on the whiteboard.
2. Calculate the proportion (or frequency) of tasters (p^2 + 2pq) by dividing the number of tasters in the class by the total number of students in the class.
3. Calculate a decimal number representing the frequency of nontasters (q^2) by dividing the number of nontasters by the total number of students.
4. In the table below, record the numbers under the appropriate headings.

 | PHENOTYPE | ALLELIC FREQUENCY
SAMPLE | TASTERS (p^2 + 2pq) | NON-TASTERS  (q^2) | p | q
Class population |  |  |  |
North-American population | 0.55 |  |  |

Q. Why is the term 2pq included in the taster phenotype?

1. Calculate the frequencies of the two alleles (p & q) in this population.  Note: The frequency q can be calculated by taking the square root of q^2. Since p + q = 1, you can calculate the frequency p of the taster allele, record your data in the table above.

1. Use the data on the North American population provided in the table to calculate the expected frequencies of the two alleles. Record these above.

#3.  How do the sample frequencies for the class compare with the frequencies for the population at large?

C. Review Questions

  1. What is an allelic frequency?
  2. If you know the allelic frequencies for a population, can you always calculate genotypic frequencies? Why or why not?
  3. If a population that is 10% AA, 20% Aa, and 70% aa, what is the frequency of allele A? What is the frequency of a?
  4. In a population that is in Hardy-Weinberg equilibrium, the frequency of the g allele is 0.4. What fraction of the population is GG? Why is it important to mention that this population is in Hardy-Weinberg equilibrium?
  5. What 5 conditions must be met for the Hardy-Weinberg equation to predict the genetic makeup of a population accurately? Why do we consider the Hardy-Weinberg prediction to be a ‘null’ hypothesis?
  6. What is genetic drift? Why can genetic drift usually be ignored in a large population but never in a small one?
  7. Explain why nonrandom mating and migration can disrupt Hardy-Weinberg equilibrium.
  8. Can a population evolve solely due to genetic drift? Explain.
  9. To illustrate Hardy-Weinberg principles, a biology teacher describes the case of a homozygous recessive fatal disease that occurs in 4 percent of a population. Students are to use the Hardy-Weinberg equation to compute p and q for this population. Is this an appropriate use of the hardy-Weinberg equation? Why or why not?

TURN IN - only the answers to the review questions!