Hardy-Weinberg Bean Lab
Objectives:
1. Understand the concepts of allele frequency, genotype frequency and phenotype frequency in a
population.
2. Understand the concept of Hardy-Weinberg population.
3. Understand the principles of genetic drift and natural selection.
4. Practice Collecting and interpreting data.
For this lab, work in groups of 2
PART 1- Hardy-Weinberg Populations
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Obtain a bag of bean, and brown paper bag.
Each type of bean has two alleles for a color gene in a population. There are two allele typesbrown and red. Assume that brown is dominant to red.
We will create a population of 50 individuals. 13 are homozygous brown, 25 are heterozygous
brown, and 12 are red.
How many alleles does each individual carry? ____________
How many total alleles are in this population? ___________
Lay out pairs of alleles on a table that represent each individual in the population.
How many brown alleles are there in the population? ____________
What proportion of the total alleles is this? (Divide answer to 3 by total alleles). This number
represents “p” in the Hardy-Weinberg equation.
P= __________. This is the allele frequency of the brown alleles in the population.
How many red alleles are there in the population? _______
What proportion of the total alleles is this? (Divide answer to 6 by total alleles). This number
represents “q” in the Hardy-Weinberg equation.
q= __________. This is the allele frequency of red alleles in the population.
The proportion of brown alleles (p) and the proportion of red alleles (q) should equal 1. Check to
see that p+q=1.
In a H-W population, random mating is assumed. That means that looks and behavior have not
effect on whether they get to mate (and pass on their alleles to the next generation). This
means a population is literally a collection of alleles (not individuals). To represent this, take all
your individuals and put them in the brown paper bag. This is your population. IT IS ALL THE
ALLELES IN A POPULATION REGARDLESS OF HOW THEY ARE ARRANGED IN INDIVIDUALS.
Without looking, draw an allele from the bag. What color did you draw? _____________
Put the allele back and shake the bag. Draw another allele. What color did you draw? _______
Repeat these two steps 8 more times (for a total of 10). What is the probability of drawing a red
allele? ___________
15 What is the probability of drawing a brown allele? ___________
16 Notice that these probabilities are the same as p and q! Allele frequencies in population are
ALSO the probability of the allele being drawn from the population!
17 Drawing TWO alleles at random is equivalent to random mating in a population. Alleles combine
at random to make the next generation. Try this by drawing 2 beans from the bag. This allele
pair represents an individual in the next generation!
18 Hardy-Weinberg says you can predict the chance of having a specific genotype drawn from the
allele pool.
a. The probability of drawing a brown allele (p) along with a second brown allele (p) would
give you a homozygous individual. So mathematically, the probability of drawing two
brown alleles is (p x p) or (p2).
19 For your example: p2= ____________
20 Likewise, the chances of drawing a homozygous red individual is the probablitiy of drawing two
red alleles (q x q) or (q2).
21 For your example: q2= ____________
22 Now let’s consider the probability of drawing a heterozygote. Here there are two probabilities
a. Either you draw a brown allele first (p) and then a red allele (q) OR
b. You draw a red allele first (q) and then a brown allele (p)
c. So mathematically, the probability is (p x q) and (q x p) or 2(p x q).
23 For your example: 2pq= ____________
24 The probability of pulling any of those combinations of alleles should equal 1 (100%). This means
that p2 + 2pq + q2 = 1. Check to see if it does.
25 SO….ASSUMING THE POPULATION MEETS ALL THE CRITERIA OF HARDY-WEINBERG, you can
accurately know what proportion of the population is homo dom, hetero, or homo recess.
26 There are 5 conditions under which populations are considered Hardy-Weinberg.
a. __________________________________________________________________
b. __________________________________________________________________
c. __________________________________________________________________
d. __________________________________________________________________
e. ___________________________________________________________________
PART 2- Genetic Drift
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Let’s see what happens if your population experiences genetic drift. What is the definition of
genetic drift? __________________________________________________________________
Put all your alleles back in the bag. You should have your initial population of 50 individuals.
Let’s suppose a tidal wave wipes out 50% of your population. To model this, reach in and blindly
grab one individual (2 alleles) and remove them. DON’T THROW THEM AWAY, SET ASIDE FOR
LATER. Record the genotype and phenotype of the individual in Table 1 below.
Continue to randomly remove individuals until only 25 remain (you have removed 25
individuals). Record the geno and pheno of every individual removed in the table.
TABLE 1: Geno and phenol of Individuals killed by the tidal wave
Individual
removed
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Color
(pheno)
Genotype
Individual
removed
Color
(pheno)
Genotype
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Calculate the values of p and q in the REMAINING population
a. p= ____________________
b. q= ____________________
Are the new values the same as the original p and q for the population? __________________
Has evolution occurred? _____________ EXPLAIN
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
HINT: remember, if your population does not equal Hardy-Weinberg, then the population is
evolving.
WHEN YOU ARE FINISHED, MAKE SURE YOU HAVE PUT BACK ALL THE BEANS INTO THE BAGGIE
AND PLACE THEM, THE PAPER BAG, AND THIS LAB SHEET INTO THE PLASTIC BAG!