Lab 8     Population Genetics

Introduction
G.H Hardy and W. Weinberg developed a theory that evolution could be described as a change of the frequency of alleles in an entire population.  In a diploid organism that has gene a gene loci that each contain one of two alleles for a single trait t the frequency of allele A is represented by the letter p.  The letter q represents the frequency of the a allele.  An example is, in a population of 100 organisms, if 45% of the alleles are A then the frequency is .45.  The remaining alleles would be 55% or .55.  This is the allele frequency.  An equation called the Hardy Weinberg equation for the allele frequencies of a population is p2+ 2pq+ q2 = 1. P represents the A allele frequency.  The letter q represents the a allele.  Hardy and Weinberg also gave five conditions that would ensure the allele frequencies of a population would remain constant. 

The breeding population is large. The effect of a change in allele frequencies is reduced.
Mating is random. Organisms show no mating preference for a particular genotype.
There is no net mutation of the alleles.
There is no migration or emigration of organisms.
There is no natural selection. Every organism has an equal chance for passing on their genotypes.
  

 If these conditions are met then no change in the frequency of alleles or genotypes will take place.

            A simple class experiment will take place to serve as model of the evolutionary process in a stimulated population.  This experiment is great in order to test a few of the basic parts of population genetics.  In the experiment the class will place a piece of paper in their mouth to see if they can taste the chemical PTC which is phenythiocarbamide.  People with the alleles AA, which is homozygous, and Aa, which is heterozygous, will be able to taste the PTC.  People that canít taste PTC are aa.  

Hypothesis
 By allowing a class to see if they can taste PTC and recording the results the Hardy Weinberg equation can be used  to determine the allele frequencies of the class.

 Materials
 The materials used in this experiment are as follows: strips of PTC test paper, paper and a pencil. 

Methods
Begin by placing a piece of the PTC test paper in your mouth.  Tasters will have a bitter taste in their mouth.  The frequency of tasters (p2 +2pq) is a found as a decimal by dividing the total number of tasters by the total number of students in the class.  The frequency of nontasters (q2 ) is found by dividing the number of tasters by the number of people in the class.  Using the Hardy Weinberg equation the frequency of  p and q can be found.  q is found by taking the square root of q2.  p is found by using the equation 1-q=p. Also calculate the frequencies of the North American population.  Finally find 2pq that represents the percentage of the heterozygous tasters in the class.  Record the results in table 8.1 

Results
Table 8.1 Phenotypic Proportions of Tasters and Nontasters and Frequencies of the Determining Alleles

 

Phenotypes

Allele Frequencies

Tasters

P2 + 2pq

Nontasters

Q2

p

Q

Class Population

#

%

#

%

 

.53

 

.47

7