The BZ220 Introduction to Evolution
POPULATION GENETICS PRACTICE PROBLEMS
Week 4
OBJECTIVES:
1. Learn how to use Hardy-Weinberg as a null model/hypothesis for evolution.
2. Understand that in natural systems, many individual loci are in Hardy-Weinberg
equilibrium, which implies that Hardy-Weinberg is useful in a practical context.
I. HARDY-WEINBERG
Hardy-Weinberg#1: Wing color polymorphisms occur regularly in butterflies. A population of
butterflies in a butterfly garden consists mostly of red-spotted butterflies (Rr) with some blackwinged butterflies (rr) and some red-winged butterflies (RR). The observed frequency of redspotted butterflies is 0.40 and the observed frequency of black-winged butterflies is 0.20. Given
this information, answer the following questions and determine whether the population is in
Hardy-Weinberg equilibrium.
a. What is the observed frequency of genotype Rr?
b. What is the observed frequency of genotype rr?
c. What is the observed frequency of genotype RR?
d. What is the observed frequency of allele R?
e. What is the observed frequency of allele r?
f. Determine the frequency that you would predict for genotype rr if the assumptions of
Hardy-Weinberg were met.
g. Determine the frequency that you would predict for genotype RR if the assumptions of
Hardy-Weinberg were met.
h. Determine the frequency that you would predict for genotype Rr if the assumptions of
Hardy-Weinberg were met.
i. Test the hypothesis that this butterfly population is in Hardy-Weinberg equilibrium by
comparing your observations to the prediction of the hypothesis. Is the population in
Hardy-Weinberg equilibrium? (Hint: it may be helpful to organize your data in a table
before reporting an answer.)
j. Which genotype(s) are in excess (the observed frequency is greater than what you would
predict) in this population?
Hardy-Weinberg #2: Your professor counted the grasshoppers in her backyard, and found there
were 300 of them. They came in three different sizes: short legged (genotype SS), medium
legged (genotype SL), and long legged (genotype LL). She sorted the grasshoppers by leg size
and found 75 short legged grasshoppers and 45 long legged grasshoppers before she became
bored and stopped counting grasshoppers. All of the remaining uncounted grasshoppers were
medium legged.
a. What are the observed genotype frequencies (i.e., the frequency of the SS type, SL type,
and LL type)?
b. What are the observed allele frequencies (i.e., the frequency of the S allele and the
frequency of the L allele)?
c. Determine the genotype frequencies that you would predict if the assumptions of HardyWeinberg were met.
d. Test the hypothesis that this grasshopper population is in Hardy-Weinberg equilibrium by
comparing your observations to the prediction of the hypothesis. Is the population in
Hardy-Weinberg equilibrium? (Hint: it may be helpful to organize your data in a table
before reporting an answer.)
e. Which genotype(s) are in excess (the observed frequency is greater than what you would
predict) in this population?
II. GENETIC DISORDERS
Genetic Disorder #1: The autosomal recessive disorder Phenylketonuria, if left untreated, can
lead to problems with brain development and seizures. This genetic disorder affects
approximately 1 in 12,000 people. Let “A” be the normal allele and let “a” be the allele for
Phenylketonuria. Assuming Hardy-Weinberg conditions:
a. What is the genotype of individuals that have Phenylketonuria?
b. What is the frequency of affected individuals?
c. What is the frequency of the disease allele?
Genetic Disorder #2: Non-classical congenital adrenal hyperplasia (NCCAH) is an autosomal
recessive disorder that affects 1 in 100 individuals in New York City (no kidding!). Let “a” be
the allele for NCCAH and “A” be the normal allele. Assuming Hardy-Weinberg conditions:
a. What is q, the frequency of “a” in New York City?
b. What is qt+100, the frequency of “a” in New York City after one hundred generations?
c. What is the frequency of individuals that are carriers (i.e., heterozygotes) for NCCAH in
New York City?