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The Hardy-Weinberg principle states that if a population is NOT evolving then the frequencies of the alleles in
the population will remain stable across generations - it is in equilibrium.
We can use the Hardy-Weinberg equation to make predictions about the relative frequency of the different
alleles (as well as the associated genotypes), even if there is population growth, as long as the five conditions
we discussed in class hold true. Think about what those five conditions are.........
Alternatively, Hardy-Weinberg equation can be a means to determine if a population is indeed evolving - that
the allele frequencies are changing and therefore NOT at equilibrium.
The trick to using the Hardy-Weinberg equation to help evaluate the frequency of a particular allele frequency
at time X, or to make a prediction about the frequency of a particular genotype/phenotype in future generations,
is to go through the math is a step-by-step manner.
Strategies for solving Hardy-Weinberg problems
a. General strategies to employ when attempting Hardy-Weinberg problems
i. Think of these problems as puzzles and then keep telling yourself that it's only a puzzle or a
game; that is, just go with the flow; stressing yourself out at any juncture in life is
counterproductive
ii. Always assume Hardy-Weinberg equilibrium unless information contradicts that assumption.
b. Work with decimals
i. Decimals typically are easier to work with than percentages, fractions, or absolute numbers.
Consequently:
1. Always convert percentage information into decimal information (i.e., frequencies; 97% 0.97)
2. Always convert fractions into decimals
c. Convert phenotypes to genotypes
i. Always look for how you might convert phenotype information into genotype information, and
then do so. Remember, solving Hardy-Weinberg problems is a game. Whenever you see
phenotype frequencies, you should start looking forward to deducing genotype frequencies
ii. Remember, frequencies must add up to one
d. Convert genotypes to alleles
i. When you have genotype frequencies, use that information to calculate allelic frequency.
Remember, again that solving Hardy-Weinberg problems is only a game. Whenever you see
phenotype frequencies, you should start looking forward to deducing genotype frequencies
ii. If you have Hardy-Weinberg equilibrium and have the frequency of one of a homozygote, then
you can calculate at least one allele's frequency as the square root of the homozygote's
frequency.
iii. Remember, frequencies must add up to one
e. Convert alleles to genotypes
i. When you have allele frequencies, you can then calculate genotype frequencies using the HardyWeinberg equation, i.e., (AA) = p2, (Aa) = 2pq, and (aa) = q2.
ii. Remember, frequencies must add up to one.
iii. Remember that you can generate genotype frequencies using the Hardy-Weinberg equation only
if Hardy-Weinberg conditions apply.
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1. Population genetics puts a mathematical approach to the study of microevolution. Define each of the terms
commonly used in population genetics.
a. population:
b. gene pool:
c. allele frequency:
2. List the five conditions that must be met by a population for genotypes to remain unchanged from generation
to generation (i.e.-a population in Hardy-Weinberg equilibrium).
a.
b.
c.
d.
e.
3. Assuming a population is in Hardy-Weinberg equilibrium, write the equation that describes genotype
frequencies.
4. In reference to the Hardy-Weinberg Principle, define the following:
a. p2
b. 2pq
c. q2
5. Work out these practice problems. Assuming H-W equilibrium, find both the allele and genotype frequencies.
a. Forty-nine percent of a population cannot taste a chemical called PTC. The presence of a dominant allele
is necessary to taste this substance. Complete the following information about the gene pool of the
population:
i. q2 =
v. 2pq =
ii. q =
iii. p =
iv. p2 =
2
b. An investigator determines, by inspection, that 4% of a population is albino. Answer the following
questions about this population:
i. q2 =
ii. This represents the percentage of the population that is
iii. What is the frequency of the recessive allele in this population? q =
iv. Considering the frequency of the recessive allele, what is the frequency of the dominant allele?
p=
v. If p = this value, then p2 =
vi. This is the frequency of the population that is
vii. The value of 2pq =
viii. This represents the frequency of the population that is
ix. What percentage of the population has normal pigmentation?
c. In Drosophilia, the allele for normal length wings is dominant over the allele for vestigial wings. In a
population of 1,000 individuals, 160 show the recessive phenotype.
d. The allele for the hair pattern called "widow's peak" is dominant over the allele for no "widow's peak." In
a population of 1,000 individuals, 360 show the dominant phenotype.
6. What is the value of using the Hardy-Weinberg equation when researching evolution in a population?
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