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BIO170 General Biology I
L. Freeman
Hardy-Weinberg and Natural Selection with M & M’s Lab
Objectives: After completing this lab, a student will:
1. Understand the concepts of allele frequency, genotype frequency and phenotype
frequency in a population.
2. Understand the concept of a Hardy-Weinberg population.
3. Understand the principles of Genetic Drift and Natural Selection.
4. Practice collecting and interpreting data.
5. Write an Introduction to a Scientific Paper
6. Use proper documentation for research.
For this lab, work in groups of 2
Part 1 – Hardy-Weinberg Populations
1) Obtain a bag of M&Ms, a paper bag and a coin.
2) Separate out the brown and red M & M’s from the bag (you could use any two colors…
I’m just choosing brown and red).
3) Each M&M represents one allele for a color gene in a population. Let’s say that there are
2 allele types – red and brown! We can also make the assumption that brown is dominant
to red.
4) We will create a population of 50 individuals. 13 individuals are homozygous brown, 25
individuals are heterozygous brown and 12 individuals are red.
5) How many alleles does each individual carry? ____________
6) How many total alleles do we need to create this population? ____________
7) Lay out pairs of alleles on a table which represent each individual in the population.
8) How many brown alleles are there in the population altogether? _________________
9) What proportion of total alleles is this? (Divide the total number of brown alleles by the
total number of alleles). This number is “p” in the Hardy-Weinberg equation.
10) p = ___________. This is the allele frequency of brown alleles in your population.
11) How many red alleles are there in the population altogether? _____________
12) What proportion of total alleles is this? (Divide the total number of red alleles by the total
number of alleles). This number is “q” in the Hardy-Weinberg equation.
BIO170 General Biology I
L. Freeman
13) q = ___________. This is the allele frequency of red alleles in your population.
14) The proportion of brown alleles (p) and the proportion of red alleles (q) should equal 1.
Check to see that your values of p + q = 1.
15) In a Hardy-Weinberg population, random mating is assumed. This means that what an
individual looks like or behaves like has no bearing on the chances that their alleles are
represented in the next generation. In essence then, the individuals do not exist! A
population is literally, a collection of alleles. To represent this, take all the alleles from
the individuals in front of you and put them in a paper bag. This bag is your population!
It is all the alleles that exist in the population regardless of how they are arranged in
individuals.
16) Without looking, draw an allele out of the bag. What color did you draw? ___________
17) Put the allele back and shake the bag. Draw another allele. What color did you draw?
_________
18) Repeat the last two steps a number of times. Do you have a feel for what the chances of
drawing a particular color allele is? What is the probability of drawing a red allele?
_______
19) What is the probability of drawing a brown allele? _________
20) Notice that these probabilities are the same as p and q! Allele frequencies in a population
are also the probability of the allele being drawn from the population!
21) Drawing 2 alleles at random is equivalent to random mating in the population. Alleles
combine at random in the population to make the next population. Try this by drawing 2
M & M’s from the bag. This allele pair represents an individual in the next generation!
22) Hardy-Weinberg says that you can predict what the chances of having any one particular
genotype being drawn from the allele pool. The chances of drawing a homozygous
brown individual is the probability of drawing a brown allele (p) and the probability of
drawing a 2nd brown allele (p). Mathematically, the probability of drawing 2 brown
alleles is (p x p) or (p2). This value also represents the genotype frequency of
homozygous brown individuals in the next generation!
23) For your example: p2 = _________.
24) Likewise: the chances of drawing a homozygous red individual is the probability of
drawing a red allele (q) and the probability of drawing a 2nd red allele (q).
Mathematically, the probability of drawing 2 red alleles is (q x q) or (q2). This value also
represents the genotype frequency of homozygous red individuals in the next generation!
25) For your example: q2 = _________.
BIO170 General Biology I
L. Freeman
BIO170 General Biology I
L. Freeman
26) Now let’s consider the probability of drawing a heterozygote. Here there are 2
possibilities. Either you draw a brown allele first (p) and then a red allele (q) or you draw
a red allele first (q) and then a brown allele (p). Mathematically, the probability of
drawing a heterozygote is (p x q) and (q x p) or (2 x p x q). This value also represents the
genotype frequency of heterozygous individuals in the next generation!
27) For your example: 2pq = _________.
28) The probability of pulling any pair of alleles should equal 1 (or 100%). This means that
p2 + 2pq + q2 = 1. Check to see if your values add up to 1.
29) What this means is that for any population that you know the allele frequencies for, you
know what proportion of the population is homozygous dominant, heterozygous, and
homozygous recessive! Of course, this is provided that the population in a HardyWeinberg population.
30) There are 5 conditions under which populations conform to Hardy-Weinberg. What are
they? (Hint: look at your lecture notes or chapter 25 in your text)
a. __________________________________
b. __________________________________
c. __________________________________
d. __________________________________
e. __________________________________
Part 2 – Genetic Drift
31) Let’s see what happens if your population experiences genetic drift. What is the
definition of genetic drift? __________________________________________
32) Recreate your initial population of 50 individuals: 13 individuals are homozygous
brown, 25 individuals are heterozygous brown and 12 individuals are red. Put all of the
alleles in a paper bag.
33) Let’s suppose a tidal wave wipes out 50% of your population. To model this, reach into
your bag and blindly grab one individual (2 alleles) and remove it from the population.
(You will need these removed alleles later in the lab so do not eat them or throw them out
yet!) Record the phenotype and genotype of that individual in Table 1 below.
BIO170 General Biology I
L. Freeman
34) Continue to randomly remove individuals until 25 have been removed. Record the
phenotype and genotype of each removed.
Table 1: Phenotype and Genotype of Individuals killed by a Tidal Wave
Individual
removed
1
2
3
13
14
15
4
5
6
7
8
9
10
11
12
Color
(phenotype)
Genotype:
Homozygous
or
heterozygous?
Individual
removed
16
17
18
19
20
21
22
23
24
25
Color
(phenotype)
Genotype:
Homozygous
or
heterozygous?
35) Calculate the values of p and q in the remaining population (hint: look back to numbers
5-14 above)
p = ____________________________
q = ____________________________
36) Are the new values the same as the original p and q for the population?
37) Has evolution occurred? Explain.
BIO170 General Biology I
L. Freeman
Part 3 – Natural Selection
38) Let’s see what happens if your population experiences natural selection. What is the
definition of natural selection? __________________________________________
39) Recreate your initial population of 50 individuals: 13 individuals are homozygous
brown, 25 individuals are heterozygous brown and 12 individuals are red. Lay the alleles
out on a table.
40) Let’s assume that the red individuals (phenotypes) are easy prey and that the brown
phenotype blends in with the environment… so is protected. In any one generation, 50%
of the red individuals fall prey to the predator (in other words they die and cannot
reproduce!)
41) To model what happens in the first generation, you want to remove ½ of the individuals
who are red.
a. How many individuals are killed? __________
b.
How many alleles must be removed from the population? ___________
c.
What alleles exactly are removed? ___________ [You can be the predator and
eat those individuals!!]
42) Now we have to replace those individuals so our population is back up to 50. Let’s say
that the remaining individuals mate randomly. To model this, put all the remaining
alleles in a bag.
43) Select two alleles for the first individual from the bag. Then select two more alleles for
the next individual from the bag. (You should have one of the following crosses: BB x
BB, BB x Bb, Bb x Bb, BB x bb, Bb x bb, bb x bb).
44) Write the cross that you have. _______________________
45) Each parent passes one of their two alleles on to their child. Decide which allele in each
parent will be “heads” in a coin toss and which will be “tails”. Flip your coin once for
each parent.
a. What color allele does mom pass on? _____________
b. What color allele does dad pass on? __________________
46) Those two alleles represent the genotype for the new child. Write the genotype of the
child. ________________
BIO170 General Biology I
L. Freeman
47) Place the parent’s alleles (not the offspring’s) back into the bag. (You should be putting
4 alleles into the bag).
48) Repeat the mating game for 5 more offspring. This should replace the 6 individuals
killed by the predator!! Write the genotypes that you added to the population.
________________
49) Your population is now at the next time period (or generation). You should have a total
of 50 individuals. How many of each genotype do you have in your population? (This
should be the original number of BB and Bb, ½ the number of original bb plus whatever
the kids were). _____________________
50) How many alleles are in your population? _____________________
51) Count the number of each color allele in your bag. _______________
52) What is your new value of p (remember this is a proportion)? _____________
53) What is your new value of q? __________________
54) What proportion of BB is there in your population? (This should be the number of BB
individuals divided by 50 (the total number of individuals)). _______________
55) What proportion of Bb is there in your population? ________________
56) What proportion of bb is there in your population? ________________
57) Does the proportion of BB = p2? ______________
58) Does the proportion of Bb = 2pq? ____________
59) Does the proportion of bb = q2? ________________
60) If the proportions are not equal to Hardy-Weinberg, the population is evolving! In this
case it should be evolving due to natural selection.
61) Repeat the experiment with your new population. Start by having ½ of the red
individuals die.
62) For each generation, keep track of the numbers of each genotype in the table below.
63) What kind of selection have you just demonstrated? (Hint – it’s one of the 3 types you
studied in Chapter 25/26.)
BIO170 General Biology I
L. Freeman
Generation
1
Homozygous brown
(BB)
13
Heterozygous brown
(Bb)
25
Homozygous red (bb)
12
Assignment:
Due one week following the lab in Google Drive. Label this BIO170_lastname_evolution
Create a document that models an introduction section of a scienitific paper describing the topics
you investigated in this lab. Here are some guidelines:
The introduction briefly introduces the topic of your study and your study organism.
The first paragraph should present the aims or goals of your study, and should provide a brief
and accurate idea of what the study is about. In general you are answering the questions:
 What is this study about?
 Why do we care?
Subsequent paragraphs (2-6) review the current literature on the topic, state the hypotheses, and
develop rationale for the research. Specific relevant information should include:
 Relevant prior studies/observation of what trying to explain
 Factors that may contribute to explanation (introducing independent & dependent
variables)
 Specific ideas of what factors may explain observation (developing hypothesis)
In the introduction, you might also wish to present the deductive reasoning that leads to the
results predicted if particular hypotheses are true.
Make sure you cite appropriate literature in this section using MLA format.