Biology 170 Exploring Biology
Population Genetics Activity
12/07
Students at each table should have:
1. A dish of blue and red pop beads is your gene pool and each bead is a gene. For these
activities we will assume that each trait has only one gene, but two copies in each body
cell.
2. Three plastic boards with depressions for beads.
3. A paper bag
4. A calculator (student provides this)
5. A coin
I. Hardy-Weinberg Equilibrium: Verifying the equation
The Hardy Weinberg equilibrium comes from an analysis of the frequency of alleles. For
each gene there are two alleles. The frequency of the most common allele is designated
as p and the least common as q. Since there are only two types of alleles, then p+q=1.
Since two gametes each with a p or q allele can combine in four possible ways as shown
in an Punnett Square, then (p+q=1)2 which can be written as: p2 + 2pq + q2 = 1.
We will determine p2 , 2pq and q2 by counting the beads (alleles) and by using the
equation to show that the equation will give you the same frequencies as counting beats
(alleles).
The red bead will be the dominant allele (p) and the blue bead will be the recessive allele
(q).
Directions for doing the activity:
1. With the three boards side by side, take beads out of the gene pool and prepare 9 pair
of blues, 9 pair of reds and 18 pair of one red and one blue beads. Line them up on row
above the line with 9 pair of blues on the left end, 9 pair of reds on the right end and 18
mixed pairs in the middle.
Our Question 1: Describe the genotypes on the boards now.
Our Question 2: Characterize the allele frequencies on the boards now.
Historical question: If selection shows preference for certain phenotypes (the optimal
form), shouldn’t the recessive alleles be selected out of the gene pool over time? The
answer why the recessive alleles did not disappear from the population was demonstrated
independently by Hardy (a mathematician) and Weinberg (a medical doctor) in1908.
2. Collect all the pairs of beads on the three boards and put them in the bag. Now have
one student select bead pairs randomly (without looking) and place them on the boards
just below the line.
3. Each two pair of beads below the line represents the parents (P1 generation) of
potential off-spring. We will assume that each union of parents will produce two offspring in the F1 generation.
4. To determine the genotype of each offspring:
a. If a “parent” is homozygous dominant (both beads red) or recessive (both beads
blue) add from the gene pool bowl the correct color of bead to each off-spring.
b. If a parent is heterozygous (one blue and one red) dominant, then you must
determine by random selection one allele over the other. To do this flip your
coin: heads is red and blue is tails. You must flip the coin for each of the two
alleles.
5. Record the genotypes of the F1 generation for your records and also record on the class
data table on the white board. Make a copy for yourself.
6. Using class data, determine for the F1 generation the values for:
p2=
2pq=
q2 =
by two methods: counting beads and by using the equation.
Does the Hard-Weinberg equation give you the same frequency value as determined by
counting beads? Why or why not?
In this trial q2 should equal 0.5. Why should it?
II. Natural Selection (non-random mating using complete dominance)
Remember: Selection is only on the phenotype. Selection cannot tell what alleles an
organism has.
1. Re-establish the P1 generation of beads above the line on the plastic boards: 9
homozygous blue bead pairs on the left end, 9 homozygous red bead pairs on the right
end and 18 heterozygous beads in the middle.
What is the value for q2 for this set up?
What is the q for this set up (called q0 for the parents’ generation)?
2. Complete Dominance (RR=Rr > rr ) in this case means that organisms with rr alleles
do not mate, therefore, put only RR and Rr bead pairs from the P1 generation in the bag.
How many pairs will this be?
3. Select bead pairs in a random manner and distribute on board below the line.
4. To determine the genotype of each offspring:
a. If a “parent” is homozygous dominant (both beads red) or recessive (both beads
blue) add from the gene pool bowl the correct color of bead to each off-spring.
b. If a parent is heterozygous (one blue and one red) dominant, then you must
determine by random selection one allele over the other. To do this flip your
coin: heads is red and blue is tails. You must flip the coin for each of the two
alleles.
5. Record the genotypes of the F1 generation for your records and also record on the class
data table on the black board. Make a copy for yourself.
Using the complete dominance class data table do the following calculations and
answer the following questions (1-5) for possible 15 points.
1. Count the beads and determine the frequencies of RR, Rr, and rr (genotype in this first
generation. (Show your work for full credit)
We will do this part in class:
Now determine the frequency of the recessive allele r as q1. How does this frequency of
the recessive allele of the first generation of offspring compare for the parents’
generation?
What will be the frequency of the recessive allele after 10 generations?
To more easily answer this question your instructor will introduce you a modified form of
the Hardy-Weinberg equation shown above.
q0
qn = _____
n+ n(q0)
Now that you are familiar with how to determine qn, for 10 generations, complete
the following two calculations and answer questions 4 and 5 based on your
calculations:
2. Using this modified Hardy-Weinberg equation given by your instructor, determine the
frequency of the recessive allele after 30 generations. (Show your work for full credit)
3. Using this modified Hardy-Weinberg equation given by your instructor, determine
the frequency recessive allele after 100generations. (Show your work for full credit)
4. Describe the change in frequency of the recessive allele from the first to the 100th
generation.
5. Will the frequency of the recessive allele ever get to zero due to the process of
complete dominance? Give support for your answer.