Anth 110,Winter 2010
Name________________________
Take-home Exercise 2: Population Genetics (20 pts)
In this exercise, you will become familiar with factors that influence changes in allele frequency
from one generation to the next through genetic drift and natural selection. This exercise will
also require you to visit a website where you will conduct simulations and record your
observations below.
#1 Simulations (10 points total)
Let’s look at some ways that gene frequencies vary in a population. For this part of the exercise
go to the website http://darwin.eeb.uconn.edu/simulations/simulations.html and follow the
instructions below.
Genetic Drift (5 points)
Go to the link entitled Genetic Drift and follow the instructions on how to use and interpret the
simulation. For all of these simulations use the default starting allele frequency of 0.5 (p: 0.5)
and the default number of maximum generations 100 (generations: 100). First, you will
experiment to see how genetic drift affects small populations. Use a population size of 10 (N:10)
and push “start” a few times (3-4) to see if a pattern emerges on the graph. Now change the
population size to 1000 (N: 1000) and push “start” a few times to see if a pattern emerges (this
works best if you look at the small population and the large population on the graph at the same
time).
Describe the pattern that you see when you compare the output (the lines on the graph)
between the small and the large population. In other words, which population (large or
small) has more variation or differences in allele frequencies (the vertical axis) through
time (the horizontal axis)?
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On which population (small or large) do you think genetic drift will have the most affect?
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Natural Selection (5 points)
Now go to the link entitled Natural Selection (you may have to follow the link if you can’t see
the simulation) and follow the instructions on how to use and interpret the simulation. You will
be investigating the strength of evolution on an advantageous dominant allele. For natural
selection you have to determine the "fitness" of each genotype. Assume that you are
investigating the fitness of free-hanging earlobes (E) versus attached earlobes (e). In this
simulation, w11 is the fitness of the EE genotype, w12 is the fitness of the Ee genotype and w22
is the fitness of the ee genotype. Larger numbers mean a genotype is more fit, relative to the
other genotypes. The vertical axis is the percentage of w11 allele (EE genotype for free-hanging
earlobes) in the population (when the vertical axis is at 1 then 100% of the population has the
w11 allele or EE genotype, when at 0 then 0% of the population has the w11 allele or EE
genotype).
Weak selection for a dominant trait:
Set the parameters of the simulation to the following: p: 0.1; w11: 1.2; w12: 1.2; and w22: 1.1
(weak selection against the ee genotype of attached earlobes).
How many generations did it take for the EE allele frequency to reach 50%?_____ What is
it after 100 generations?________
Describe the pattern of change of the EE allele frequency (straight line up at a steep angle,
exponential, etc.)_________________________________________________________
Strong selection for a dominant trait:
Set the parameters of the simulation to the following: p: 0.1; w11: 1.2; w12: 1.2; and w22: 0.8
(strong selection against the ee genotype of attached earlobes).
How many generations did it take for the EE allele frequency to reach 50%?_____ What is
it after 100 generations?________
Describe the pattern of change of the EE allele frequency relative to the first experiment
(weak selection)________________________________________________________________
Why is deleterious recessive allele is still there, even when the selection against the allele is
very strong?__________________________________________________________________
Strong selection for a recessive trait:
Set the parameters of the simulation to the following: p: 0.1; w11: 1.2; w12: 0.8; and w22: 0.8
(strong selection for the EE genotype, only - as if E was recessive and was only expressed in
homozygotes).
How many generations did it take for the EE allele frequency to reach 50%?_____ What is it
after 100 generations?________
Describe the pattern of change of the EE allele frequency______________________________
How does this result compare to the one you got with the strong selection for a dominant trait?
What’s going on?
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#2 Hardy-Weinberg Formula (10 points total)
The Hardy-Weinberg Formula can be used to determine the frequencies of alleles of a particular
gene in a given population and predict allele frequencies in future generations.
The law requires the following assumptions:
the population mates randomly
no mutations occur
no selection occurs (no influence of “fitness”).
no migration (no gene flow)
large population size (no genetic drift)
For a given gene A with alleles frequencies p (dominant) and q (recessive) in a parental
population the next generation will be in equilibrium (the gene frequencies p and q will not
change over time).
p = the frequency of the dominant allele
q = the frequency of the recessive allele
AND
p2 = the frequency of the homozygous dominant individuals
2pq = the frequency of the heterozygous individuals
q2 = the frequency of the homozygous recessive individuals
Hardy-Weinberg equilibrium is represented by the following equations:
p2 + 2pq + q2 = 1
p+q=1
How to solve Hardy-Weinberg problems:
How to find the frequency of alleles given phenotype frequency.
1. Find the number of recessive homozygous individuals (those with the recessive trait)
2. Find (count) the population size (the total number of individuals).
3. Divide the number of individuals with the recessive trait by the total number of individuals.
This is q2, the frequency of the homozygous individuals (expressed as a decimal, i.e.
70%=0.70).
4. Take the square root of q2 to get q.
5. Subtract q from 1 to get p.
6. Now that you know p and q, you can solve for 2pq (2*p*q) and p2 (p*p).
Solve the following (5 pts)
In a class of 50 students, 18 students had no widow’s peak and 32 had a widow’s peak. Having a
widow’s peak is dominant (W) and no widow’s peak is recessive (w). Assume that the
population is in Hardy-Weinberg equilibrium and solve for the following (show your work!):
a.
What is w, the frequency of the recessive allele?
b.
What is W, the frequency of the dominant allele?
c.
What is p2, the frequency of the homozygous dominant individuals?
d.
What is 2pq, the frequency of the heterozygous individuals?
e.
What is q2, the frequency of the homozygous recessive individuals?
Solve the following (5 points)
Of 100 people in our town (our population), 36 have a counter-clockwise hair whorl (h). Ten of
the people in this population decided to go on an ocean cruise together, and they were stranded
on a deserted island for 20 years. Seven of the people stranded have a counter-clockwise hair
whorl (h), a genotype frequency much different from the original population. Use the the HardyWeinberg formula is used to calculate how each population has changed (show your work!). HH,
Hh=clockwise hair whorl, hh= counterclockwise hair whorl.
Island Population size:
%hh=
%HH, Hh=
h=
H=
HH+2Hh+hh=1
(
)2+2(
)(
)+(
2
) =1
+
+
Therefore:
HH=
% Hh= % hh=
)2=1
New Original Population size:
%hh=
%HH, Hh=
h=
H=
HH+2Hh+hh=1
(
)2+2(
)(
)+(
=1
%
+
HH=
+
% Hh=
% hh=
=1
%
This is an example of what kind of evolutionary force?_______________________________.
What are the new percentages of each genotype in each population?___________________
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In terms of dominant and recessive traits, what will be the percentages of phenotypes of
each population?_______________________________________________________________
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