Busy Bees in AP Biology...
by Leanne Hinrichs
The AP Biology class at Chadron High has been busy as bees! We recently completed a lab on Population Genetics and Evolution. To do this lab, we had to first have a general idea about the Hardy-Weinberg Law of Genetic Equilibrium. In 1908, G.H. Hardy and W. Weinberg suggested that evolution could be viewed in terms of changes in allele frequencies. The frequency of the possible diploid combination of the alleles (AA, Aa, aa) is expressed as p² + 2pq + q² = 1.0. Certain conditions, however, must be present for this to apply. These include a large breeding population, random mating, no allele mutation, no immigration or emigration, and no selection occurring naturally. The equilibrium hardly ever works out this perfectly in nature, as there is no way to ensure all of these conditions. Since, by this point, we had a decent understand of these principles, we were able to begin the experimenting.
Our first exercise involved estimating allele frequencies for a specific trait within a sample population. Using our class as a random population, we isolated our variable to be the ability to taste phenylthiocarbamide (PTC). Everyone tasted both a control strip of paper and a PTC strip of paper. In our class, six out of ten people could taste the bitterness of PTC. The six tasters were all either homozygous dominant (AA) or heterozygous (Aa) for the allele. Since our class made sure to meet all the conditions, we were able to insert these data into the Hardy-Weinberg equilibrium.
p² + 2pq + q² = 1.0
(.36) + (.16) = .52
1.0 – (.52) = .48
Next, we changed gears to test an ideal Hardy-Weinberg Population. We let our class represent a breeding population. We then made sure every class member received four card (two with A, two with a). The four cards represent the products of meiosis. Each parent contributed a haploid set of chromosomes to the next generation. We made sure we kept it random, and went on to “mate” with four other people’s sets of cards. We recorded this data and moved on to an example that represented selection within the population. This process was done basically the same as the first population, except that every “aa” offspring did not reproduce. This thinned out the population of homozygous recessive and made it more common to find heterozygous and homozygous dominant. Next, we did a test of heterozygote advantage. Everything was kept the same as in Round II, except that in the event that the offspring was AA, a coin was flipped. If it was heads up, the individual does not survive; if tails, the individual lives. In this trial, selection did not favor either homozygous condition, but rather the heterozygous. This represents stabilizing selection. This concluded our experiment, and we were able to evaluate what we had learned. We began by learning about the Hardy-Weinberg Law of Equilibrium and the conditions needed to satisfy it, then moved on to testing the reproduction in a Hardy-Weinberg Population. Following this, we put spins on the ideal population by introducing selection and heterozygote advantage. What a great learning experience!
