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 can’t 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

77.78

2

22.22

North American Population

55

45

.33

.67

1.      What is the % of heterozygous tasters 2pq in your class? 49.82%

2.      What % of the North American population is heterozygous for the taster trait? 44.15% 

Case I Ideal Hardy Weinberg Populations

Introduction
In this experiment the entire class will represent an entire breeding population.  In order to ensure random mating, choose another student at random. The class will simulate a population of randomly mating heterozygous individuals with an initial gene frequency of .5 for the dominant allele A and the recessive allele a and genotype frequencies of .25 AA, .50 Aa and .25 aa.  Your initial genotype is Aa.  Record this on the data page.  Each member of the class will receive four cards.  Two cards have a and two cards have A.  The four cards represent the products of meiosis.  Each “parent” contributes a haploid set of chromosomes to the next generation.  

Hypothesis
By conducting the experiment under ideal conditions we will be able to show an ideal Hardy Weinberg population. 

Materials
The materials used in this experiment are as follows:  cards labeled A and a, a pencil and a piece of paper. 

Methods
Begin the experiment by turning over the four cards so the letters are not showing, shuffle them, and take the card on top to contribute to the production of the first offspring.  Your partner should do the same.  Put the two cards together.  The two cards represent the alleles of the first offspring.  One of you should record the genotype of this offspring in the Case I section on page 98.  Each student pair must produce two offspring, so all four cards must be reshuffled and the process repeated to produce a second offspring.  Then, the other partner should record the genotype.  The very short reproductive career of this generation is over.  Now you and your partner need to assume the genotypes of the two new offspring.  Next, the students should obtain the cards requires to assume their new genotype.  Each person should then randomly pick out another person to mate with on order to produce the offspring of the next generation.  Follow the same mating methods used to produce offspring of the first generation.  Record your data.  Remember to assume your new genotype after each generation.  The teacher will collect class data after each generation.   

Results

Case I

 

AA