Activity 8A: Estimating Allele Frequencies for a Specific Trait within a Sample Population Using the class as a sample population, the lab group will estimate the allele frequency of a gene controlling the ability to taste the chemical PTC (phenylthiocarbamide). A bitter-taste reaction to PTC is evidence of the presence of a dominant allele in either the homozygous condition (A/A) or the heterozygous condition (A/a). The inability to taste the chemical at all depends on the presence of homozygous recessive alleles (a/a). Activity 8B: A Test of Hardy-Weinberg Equilibrium
In this activity, the entire class will simulate a population of randomly mating individuals. Choose another student at random (for this simulation, assume that gender and genotype are irrelevant to mate selection.) The population begins with a frequency of 0.5 (50%) for the dominant allele A and also 0.5 (50%) for the recessive allele a. The lab group’s initial genotype is A/a. Record this on the Data Sheet. The lab group has four cards: each represents a chromosome. Two cards (chromosomes) will have allele A and two cards will have allele a. The four cards represent the products of meiosis. Each “parent” will contribute a haploid set of chromosomes to the next generation. Activity 8C: Selection
In nature, not all genotypes have the same rate of survival; that is, the environment might favor some genotypes while selecting against others. An example is human sickle-cell anemia, a disease caused by a single gene mutation. Individuals who are homozygous recessive (a/a) often do not survive to reach reproductive maturity. In this simulation, the lab group will assume that the homozygous recessive individuals never survive (100% selection against), and that heterozygous and homozygous dominant individuals survive 100% of the time.
Activity 8D: Heterozygote Advantage
From Exercise 8C, it is easy to see that the lethal recessive allele rapidly decreases in the population. However,...