Population Genetics Lab
The Hardy Weinberg Principle
Background: When more than one allele for a gene exists within a population, the frequency of the
alleles tends to remain constant. Several factors can cause the frequency of an allele to change,
including selection pressure (natural or artificial) and interbreeding between previously isolated
populations. Nevertheless, frequencies tend to remain constant. If a population of organisms
contains two alleles for a trait (and only two), then we can say that the frequency of the two combine
to give the total. Hardy and Weinberg (a biologist and a mathematician) came up with a simple
formula for such a situation.
Let “p” equal the frequency of the dominant allele, and let “q” equal the frequency of the recessive
allele
p+q=1
(The p and q alleles make up 100% of the alleles for a given trait within a population.)
Since all individuals have two alleles, and they can have 2 p’s, 2 q’s, or one of each, then we can say
that the diploid individual will influence the population by the frequency of both alleles. We can
therefore square the above equation on both sides to produce the following “didplid” version.
(p + q)2 = 12
=
p2 + 2pq + q2 = 1
(Homozygous dominant + heterozygous + homozygous recessive = 100% of the individuals in a
population with respect to a given trait.)
PTC is a chemical which is bitter (and quite harmless) to some individuals, and completely
tasteless to others. The ability to taste PTC is controlled by a dominant allete, T, meaning that
homozygous dominant and heterozygous individuals are ‘tasters’, while homozygous recessive
individuals are ‘non-tasters’.
Objective: To determine the allele frequencies of the dominant and recessive alleles for tasting PTC
within a class, and see how that compares to the frequencies in the human population. Your teacher
knows that ‘secret’ information.
Material: A piece of paper with PTC on it.
Procedure: Put a piece of PTC paper in your mouth.
Move it around a bit.
Put it in the trash.
Contribute your phenotype to the class. (taster = T_, non-taster = tt)
Use the class tally to determine the frequency of both alleles using the
equations provided by Hardy and Weinberg.
**
The non-tasters are tt (q2), the tasters are TT or Tt (p2 or pq).
Tabulate data, and show all calculation on another sheet.