AP Bio Lab 8: Population Genetics and Evolution
Mrs. Dietz
Rachel Butch
Laura Kelly
Pd: 7
4/6/11
Introduction:
The concepts surrounding this lab include the Hardy-Weinberg law of genetic equilibrium and
the relationship between evolution and changes in the frequency of alleles. The Hardy-Weinberg
equilibrium equation is the mathematical description of the fact that allele and genotype frequencies
remain constant in a random-mating population in the absence of inbreeding, selection, or other
evolutionary forces.
The lab required the ability to calculate allele frequencies in a population, given the
number/percentage of phenotypes. The lab also focused on the study of natural selection’s effect on
allele frequencies in a population.
Questions:
Methods and Procedure:
Exercise 8A:
Lab exercise A tested the class for the PTC tasting allele. If a student had a dominant allele by
either being homozygous dominant or heterozygous, they would present a reaction to the bitter taste of
the paper. The lab then directed to determine the allele frequencies of the classroom sample
population using the Hardy-Weinberg equation. This was done by calculating the percent of tasters in
the class by dividing the number of students who carried the dominant allele by the total number of
students. Data on the phenotypic percentiles of tasters and non-tasters in North America was also given
to repeat the calculations and determine the allele frequencies of the continent.
Exercise 8B:
Exercise B was made up of four case studies that involved the class representing a breeding
population. Gender and genotype were irrelevant for this simulation.
In case I the class simulated a population of randomly mating heterozygous organisms. The
initial gene frequency of this population is 0.5 for the dominant allele “A” and recessive allele “a;” and
0.25 for genotype AA; 0.5 Aa and 0.25 aa. Students then randomly found partners and chose from their
four cards (two A and two a) and created genotypes for their offspring, the F1 generation. Both partners
then replace their cards with that of the F1 generation and repeat the process until the F5 generation is
reached. At the end of the procedure, the class frequency for the F5 generation genotypes were taken.
Using this information the students then calculated the allele frequencies of this hypothetical cohort
using the given formula.
In case II, the class investigated a more realistic case. In nature, all genotypes do not have the
same rate of survival. For this exercise, any individual with a homozygous recessive genotype did not
survive to reproduce, meaning there was 100% selection against the aa genotype. The exercise was
conducted the same as case I. however, when an offspring with the genotype of aa appeared, the
parents had try again until they produced a viable descendant. The procedure repeated until the F5
generation. The class genotype frequencies were then documented and the allele frequencies were then
calculated using the given equation.
Case III exemplified what occurs in a population in which the heterozygote genotype has an
advantage. The case of sickle cell anemia was used again in this case. Humans who are heterozygous for
the disease have a greater immunity to malaria than those with a homozygous dominant genotype. The
simulation began with the same initial genotype, and same consequences for a homozygous recessive
offspring. However, this time when a homozygous dominant offspring appeared, a flip of a coin
determined if the offspring lived or died. The parents had to try until a viable offspring was created. This
procedure was repeated until the F15 generation. The class genotype frequencies were then taken for
the F5, F10 and F15 generations. The given formula was used to calculate the allele frequencies of the
population.
Case IV explored genetic drift. The procedure of case one was repeated except this time the
class was divided to create smaller populations.
Answers:
Exercise 8A:
1. 38%
2. 44%
Exercise 8B:
Case I:
1.
2.
3. The class is not follow the assumption of a utilizing a very large population necessary for HardyWeinberg equilibrium. The class size was relatively small.
Case II:
1. Initial frequencies: p= 0.5; q=0.5
Simulation frequencies: p= 0.8; q= 0.2
2. The class population was not large enough to follow the Hardy-Weinberg assumptions. There is
also selection against the recessive allele.
3. The number of recessive alleles in the population would decrease. Therefore the frequency of p
alleles would increase.
4. No, heterozygous organisms would still mate, potentially passing on a recessive gene, creating
heterozygous offspring.
Case III:
1. Case I: p 0.275, q 0.725
Case II: p 0.8, q 0.2
Case III: F5: p 0.7, q 0.3 F10: p 0.85, q 0.15 F15: p 0.75, q 0.25
2. No because of the increased number of heterozygote survival.
3. Recessive genes can resurface and serve a helpful function.
Case IV:
1. The original frequency of