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INVESTIGATION
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C2 Allele Frequency and Evolution
Key Question: What happens to the frequency of an allele in a population over time?
This investigation teaches an advanced topic that builds
on concepts explored in Investigations B5 through B8.
Here, students learn to use a mathematical model used
by scientists to track how alleles change over time in
populations of the same species. They determine the
genome of a Crazy Creature and identify the alleles it
carries for two traits. Then they study those traits in two
different populations of Crazy Creatures. Students will
model how favorable alleles are passed on to offspring
and how allele frequencies within one of the populations
change over time. Finally, they compare the allele
frequencies for the two traits in each population.
GETTING STARTED
Time 150 minutes
Setup and Materials
1. Make copies of investigation sheets for students.
2.
3. Review all safety procedures with students.
Materials for each group
yy Crazy Chromosomes set
Learning Goals
✔✔Explain how changes in allele frequency over time are
an indication that evolution is occurring.
✔✔Calculate allele frequencies for populations given the
frequency of homozygous recessive individuals.
✔✔Evaluate the importance of genetic variation to
the survival of a species when changes in the
environment occur.
Watch the equipment video.
Online Resources
Available at curiosityplace.com
yy Equipment Video: Crazy Chromosomes
yy Skill and Practice Sheets
yy Whiteboard Resources
yy Animation: Galapagos Finches
yy Science Content Video: Heredity
yy Student Reading: Traits
NGSS Connection This investigation builds conceptual understanding and skills for the following performance expectation.
HS-LS4-3. Apply concepts of statistics and probability to support explanations that organisms with an advantageous
heritable trait tend to increase in proportion to organisms lacking this trait.
Science and Engineering Practices
Analyzing and Interpreting Data
Disciplinary Core Ideas
LS4.B: Natural Selection
Crosscutting Concepts
Patterns
LS4.C: Adaptation
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Allele Frequency and Evolution
Vocabulary
adaptation – an inherited characteristic that enhances
an organism’s chance for survival in its current
environment
allele frequency – a number that relates the occurrence
of a particular allele for a gene within a population
crossover – a physical exchange of chromosome
segments that most commonly occurs early in the first
division of meiosis
evolution – a change in the genetic makeup of a
population over time
fitness – the ability of an organism to survive and
reproduce to pass its genes on to the next generation
genetic variation – the diversity of alleles in a population
of organisms
Hardy-Weinberg formulas – mathematical relationships
that are used to calculate allele frequencies in a
population of organisms
linked genes – genes that are found on the same
chromosome pair
genes and traits, they discovered patterns of inheritance
that did not match Mendel’s results. This led to the
discovery of linked genes. Linked genes are found on
the same chromosome pair and do not follow the law of
independent assortment.
In linked genes, recombination can occur through a
process called crossover. In crossover, segments of DNA
are exchanged between non-sister chromatids of
homologous pairs. The diagram below shows how
crossover can occur. The chance of crossover happening
is directly proportional to the distance of a gene from the
centromere. The further from the centromere the gene is
located, the greater the chance of crossover occurring. If a
test cross is performed using genes that are known to be
linked, and some of the offspring do not resemble either
parent, we can deduce that crossover has occurred.
Because crossover does not happen in every line of
gametes, the ratios are different than the ratios in
non‑linked genes. Students will discover that usually,
most offspring resemble at least one parent and only a
smaller number bear no resemblance to their parents.
mutation – a change in the hereditary material of
an organism
Greater chance of
crossover
natural selection – the process by which organisms with
favorable adaptations survive and reproduce at a higher
rate than those with less-favorable adaptations
Lesser chance of
crossover
non-linked genes – genes that are found on different
chromosome pairs and are inherited independently from
each other
recombination – a mixing of alleles that occurs
during meiosis
BACKGROUND
The law of independent assortment was derived from
Gregor Mendel’s work with pea plant traits. Although he
knew nothing about genes or chromosomes, Mendel
deduced that there was a mixing force that caused
alleles (he called them “factors”) to segregate into
different gametes independently of each other. It turns
out that Mendel’s data was collected from non-linked
genes: genes found on different chromosome pairs that
segregate independently. But as other scientists studied
Greater chance of
crossover
Crossover
A mutation is a change in the hereditary material of an
organism. A mutation may lead to different alleles of a
gene which, in turn, lead to variations of a trait. Mutated
alleles may cause favorable or unfavorable traits to
surface. An adaptation is an inherited trait that helps an
organism survive. Adaptations include body structures
that help an organism feed, move around, and protect
itself. Adaptations are inherited; therefore, they must be
carried on genes. Some mutations are harmful because
they cause genetic disorders. Mutations may also be
helpful because they contribute to genetic variation.
Genetic variation refers to the diversity of alleles in a
population, and ensures that a population has a better
chance of survival should the environment change.
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Imagine a population of brown squirrels that has
a single gene that determines fur color. A mutated
allele causes white fur instead of the usual brown fur.
The squirrels with brown fur can hide from predators
more easily than squirrels with white fur. Most of the
squirrels that survive to reproduce are brown. This
example illustrates the process of natural selection.
Since brown fur is a favorable adaptation, the allele for
brown fur is selected over the allele for white fur. In
natural selection, organisms with favorable adaptations
survive and reproduce. They pass favorable adaptations
on to offspring. Over many generations, the alleles for
favorable adaptations increase in the population.
The Hardy-Weinberg formulas can be used by scientists
to determine whether evolution has occurred. Using
the equations, any changes in the allele frequencies
in a population over time can be detected. An allele
frequency is a measure of the occurrence of a particular
allele of interest in a population. The Hardy-Weinberg
principle states that if no evolution is occurring, an
equilibrium of allele frequencies will remain in effect
in each succeeding generation of sexually reproducing
individuals. In order for equilibrium to remain in effect
(or, in order to conclude that no evolution is occurring),
the following conditions must be met:
bo
1. No mutations can occur, so that new alleles do not
enter the population.
2. No migration of individuals into, or out of, the
population can occur.
3. Random mating must occur (individuals must pair
by chance).
4. The population must be large.
Hardy-Weinberg Formulas
p + q = 1, where:
p = frequency of the dominant allele in the population
q = f requency of the recessive allele in the population
Fitness refers to the ability of an individual to survive,
reproduce, and contribute its alleles to the next
generation in the population. The term "Darwinian
fitness" is often used to distinguish this term from
physical fitness. Over generations, the alleles with higher
fitness become more common in the population.
and
p2 + 2pq + q2 = 1, where:
p2 = frequency of homozygous dominant individuals
2pq = frequency of heterozygous individuals
Natural selection is the driving force behind evolution. In
evolution, the alleles of a population change over time
as favorable phenotypes are selected over unfavorable
phenotypes. Through evolution, the genetic makeup of
the population changes over time. Eventually, evolution
leads to the formation of new species from a common
ancestor. The new species are genetically different from
each other and they can no longer interbreed.
q2 = frequency of homozygous recessive individuals
bo
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Allele Frequency and Evolution
5E LESSON PLAN
Engage
Take your students outside into the schoolyard. Have
them bring their notebooks. Ask students to find a single
organism, either an animal or plant, and have them
make a sketch of the organism or take a digital photo to
print out and paste into their notebook later. Ask them
to make a list of the characteristics of the organism.
Then ask them to identify three characteristics from
their list and describe how those characteristics may be
adaptations to the organism’s environment. For example,
a plant’s fuzzy leaves may help the plant trap moisture in
a dry environment.
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Complete Investigation C2, Allele Frequency and Evolution.
In this investigation, students apply what they have
learned about genes, mutations, and the processes of
meiosis and fertilization, to studying the process of natural
selection and evolution. They will study two fictional
populations of organisms (Crazy Creatures) and compare
allele frequencies using the Hardy-Weinberg formulas.
They will deduce whether evolution is occurring in one of
the populations based on their data.
Explain
Revisit the Key Question to give students an opportunity
to reflect on their learning experience and verbalize
understandings about the science concepts explored in
the investigation. Curiosityplace.com resources, including
student readings, videos, animations, and whiteboard
resources, as well as readings from your current
science textbook, are other tools to facilitate student
communication about new ideas.
Science Content Video
Heredity
Animation
Galapagos Finches
Elaborate
The concept taught in this investigation, allele frequency,
allows scientists to study genes at a population level. In
previous investigations, students used Punnett squares
to study the genes of individuals. The development
of the Hardy-Weinberg formulas was an important
milestone in the science of genetics and evolution.
Before Hardy and Weinberg, it was thought that
dominant alleles must, over time, inevitably drive
recessive alleles out of existence. This incorrect theory
was called “genophagy” (literally “gene eating”).
According to this wrong idea, dominant alleles always
increase in frequency from generation to generation.
Hardy and Weinberg were able to demonstrate with their
equation that dominant alleles can just as easily decrease
in frequency. Use this example to illustrate an important
idea about the process of science: that scientific ideas
change and evolve over time as new information is
acquired.
Evaluate
yy D
uring the investigation, use the checkpoint
questions as opportunities for ongoing assessment.
yy A
fter completing the investigation, have students
answer the assessment questions on the Evaluate
student sheet to check understanding of the concepts
presented.
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Name ____________________________________________ Date ________________________
Table 1: Creature genome
C2 Allele Frequency and Evolution
Materials:
✔ Crazy Chromosomes set
What happens to the frequency of an allele in a population over time?
✔ Allele coins
Gene color
Trait
Allele on
chromosome 1
of the pair
Black
Skin Color
T
T
TT
red
Light Purple
Leg
t
t
tt
long
Light blue
Foot
T
t
Tt
webbed
White
Arms
t
T
Tt
long
Red
Hands
t
t
tt
claws
Yellow
Eye color
t
T
Tt
red/green
Dark Purple
Eyebrows
T
T
TT
unibrow
Orange
Beak
T
T
TT
trumpet
Gray
Ears
t
t
tt
mouse
Green
Antenna length
T
t
Tt
long
Dark Blue
Antenna shape
t
T
Tt
knob
Allele on
chromosome 2
of the pair
Genotype for trait Phenotype for trait
Chromosome pair #1
Evolution is a change in the genetic makeup of a population over time, driven
by a process called natural selection. All organisms compete for limited resources such as food and shelter.
In natural selection, individuals with favorable phenotype variations will be more likely to survive and pass
the alleles for those phenotype variations on to their offspring. Eventually, the favorable phenotype becomes
prevalent in the population. Imagine a population of Crazy Creatures that lives on the mainland where there
are plenty of food sources. This is the original population. A huge storm carries some of that population to a
secluded island called Walnut Island. The only food source for this population is rock hard walnuts that fall
out of the trees. Will certain phenotypes be more favorable than others in this new environment? One hundred
years later, how might we determine if evolution is occurring in the new population?
Chromosome pair #2
Determining the genome of a creature in the original population

Let’s start the investigation by studying the original population of Crazy Creatures that existed before the
storm. Working by yourself, you will randomly determine the genome of an individual in the population.
Follow the steps below to determine your creature’s genome.
1. Take out the small plastic bag with the coins in it. You will need to share these coins with other members
of your group since everyone is going to flip for their own creature.
a. Find the blue coin that has a T on one side and a t on the other. Use this coin to flip for the first
chromosome in each pair. Find the green coin with a T on one side and a t on the other. Use this coin to
flip for alleles on the second chromosome in each pair. We won’t worry about the sex of our creatures for
this investigation, and will focus only on genes that code for other traits.
Yellow-green
Tail
t
t
tt
none
Pink
Wings
T
T
TT
none
2. The first trait you will flip for is skin color. Flip the blue coin and record the result in Table 1 in the skin
color row. Flip the green coin and record the result in the skin color row as well. Pass the coins around in
your group so everyone has a chance to flip for the trait.
3. Record the genotype for your creature’s skin color in the genotype column. An organism’s genotype is
the set of alleles for a particular gene found in the organism’s genome. To record the genotype, simply
write the letters in columns 2 and 3. In genetics, the capital letter is always written first, regardless of
which chromosome it is located on. For example, if you recorded a t in column 2 and a T in column 3, the
genotype for that trait would be Tt.
4. Flip the coins for the rest of the traits. Record the alleles on each chromosome in each pair as well as
the genotypes for all of the traits. Share the coins with other members of your group so everyone can
determine the genotypes for their creatures.
5. Use Table 2 to decode your creature’s genome and fill in the phenotype column of Table 1 for each trait.
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Crazy Chromosomes
Guiding the INVESTIGATION
 Determining the genome of a creature
in the original population
If students have completed Investigation B1, they
will be familiar with the process of flipping coins to
determine the genotype and phenotype of a Crazy
Creature. You may have them use their data from that
investigation.
Unlike in earlier investigations, each individual
student should flip for his or her own creature for
this investigation. This is because we want a larger
starting population for the rest of the investigation.
Students flip the allele coins that come with the
Crazy Chromosomes set and take turns using them.
Alternatively, you can have them use regular coins,
with heads representing the dominant allele and tails
representing the recessive allele.
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Table 2: Genotypes and phenotypes for Crazy Creatures
Trait
1. Sex
Genotypes and phenotypes
XX – female
XY – male
2. Skin color
TT – red Tt – purple
tt – blue
3. Leg
TT – short Tt – short
tt – long
4. Foot
5. Arms
6. Hands
7. Eye color
TT – webbed
Tt – webbed
TT – long
Tt – long
tt – talon
tt – short
TT – paws Tt – paws
tt – claws
TT – red Tt – one red and one green
TT – unibrow
tt - green
Tt – unibrow
tt – separate
9. Beak
TT – trumpet Tt – trumpet
tt – crusher
10. Ears
TT – elephant
8. Eyebrows
11. Antenna length
TT – long
12. Antenna shape
TT – knob
13. Tail
TT – long
14. Wings
TT – no wings
Tt – elephant
Tt – long
Tt – knob
Tt – short
tt – mouse
tt – short
tt – star
tt – none
Tt – no wings
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Calculating allele frequencies using the Hardy-Weinberg formulas

To determine if evolution is occurring, we can start by determining the allele frequencies for alleles in the
original population, then comparing those frequencies with the population on Walnut Island today (100 years
after the storm that carried them there). If there are differences in the allele frequencies, we will have evidence
that favorable phenotypes are being selected by the environment (i.e., natural selection is occurring).
The Hardy-Weinberg formulas can be used by scientists to determine whether evolution has occurred. Using
the equations, any changes in the allele frequencies in a population over time can be detected. The law states
that if no evolution is occurring, then equilibrium of allele frequencies will remain in effect in each succeeding
generation of sexually reproducing individuals. In order for equilibrium to remain in effect (or, in order to
conclude that no evolution is occurring), the following conditions must be met:
1. No mutations can occur, so that new alleles do not enter the population.
2. No migration of individuals into, or out of, the population can occur.
3. Random mating must occur (individuals must pair by chance).
4. The population must be large.
Obviously, all of these conditions cannot be met in real-life situations. But we can use the formulas to take
a “snapshot” of the allele frequency for a gene in a population. By comparing the allele frequencies of the
original population and the present-day Walnut Island population, we can determine whether evolution is
occurring on the island. If there is a difference in the allele frequency between the two populations, then all
of the Hardy-Weinberg conditions have not been met, and evolution may be occurring on the island. If the two
frequencies are the same, then we can hypothesize that evolution is not occurring.
There are two Hardy-Weinberg formulas.
tt – wings
Hardy-Weinberg Formulas
p + q = 1, where:
p = frequency of the dominant allele in the population
q=
frequency of the recessive allele in the population
and
p2 + 2pq + q2 = 1, where:
p2 = frequency of homozygous dominant individuals
2pq = frequency of heterozygous individuals
q2 = frequency of homozygous recessive individuals
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SCIENCE AND MATH
Using the Hardy-Weinberg formulas If your
students are good with algebra, they will quickly
understand the Hardy-Weinberg formulas. These
formulas are called “equilibrium formulas” because all
of the variables add up to 1. In this equation (p² + 2pq
+ q² = 1), p is defined as the frequency of the dominant
allele and q as the frequency of the recessive allele for
a trait controlled by a pair of alleles (T and t). In other
words, p equals all of the alleles in individuals who
are homozygous dominant (TT) and half of the alleles
in individuals who are heterozygous (Tt) for this trait
in a population. Likewise, q equals all of the alleles in
individuals who are homozygous recessive (tt) and
the other half of the alleles in individuals who are
heterozygous (Tt).
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Go through the example in Part 2 of the investigation
as a class to make sure students have a grasp of how
to use these formulas. If they are having trouble, they
will need practice before continuing. Here is another
example to use in addition to the one presented in the
investigation.
Within a population of moths, brown color (B) is
dominant to white color (b). If 30% of the population is
white, what is the frequency of homozygous dominant
and heterozygous individuals in the population?
bb = q 2 = 0.30
q = 0.30 = 0.55
p = 1 − 0.55 = 0.45
p 2 = (0.45) = 0.20 = BB
2
2 pq = 2(0.45)(0.55) = 0.50 = Bb
q = 0.30 = bb
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So, if an individual has claws, we know that its genotype is hh. Similarly, if the individual has a crusher beak,
we know its genotype is bb. Follow the steps below to calculate the allele frequencies for the hand and beak
genes for the class population.
Here is an example of how to use the Hardy-Weinberg formulas.
You have sampled a population in which you know the frequency of individuals out of the total population
is 0.24 (which is equal to 24%) homozygous recessive for a trait (tt). Use the Hardy-Weinberg formulas to
calculate the frequency of the T and t alleles in this population. Then find the frequency of each possible
genotype (TT, Tt, tt).
1. Your teacher will guide the class in filling out Table 3. Use this data to calculate the allele frequencies
for the hand and beak genes. The allele frequencies are equal to the number of creatures with the allele
divided by the total number of creatures in the population.
1. What do you know? The value of q2 = the frequency of the population that is tt (0.24).
2. Use the Hardy-Weinberg formulas to calculate the frequency of the alleles of each gene and record the
frequencies in the last two columns of Table 3.
2. What do you want to find out? The allele frequencies of T and t in the population, and the frequencies
of each possible genotype (TT, Tt, and tt).
Table 3: Frequency of homozygous recessives in original population
3. Formulas to use: p + q = 1 and p2 + 2pq + q2 = 1
4. Solution:
Trait
To find the frequency of t, simply take the square root of 0.24, since q2 = tt:
0.24 = 0.49
Since p + q = 1 and we now know the value of q, we can calculate p using this relationship:
Number of
Frequency of
homozygous recessive homozygous recessive
individuals
individuals
Total
Population
24
24
Hands
3
5
0.13
0.21
Beak
p + 0.49 = 1
p = 1 – 0.49 = 0.51
Frequency of
recessive allele
(h or b)
Frequency of
dominant allele
(H or B)
0.36
0.46
0.64
0.54
Modeling reproduction on Walnut Island

Since p2 + 2pq + q2 = 1, and we know the values of p and q, we can easily determine the frequencies of
each genotype in this population.
The storm blew a random sample of the mainland population to Walnut Island. In this part of the investigation,
we will model reproduction in the P1 generation to produce the F1 offspring.
p = 0.51 and q = 0.49, so:
Frequency of TT = (0.51)2 = 0.26
Frequency of Tt = 2(0.51)(0.49) = 0.50
1. Randomly choose the genome of the creature in your group that you choose to be a parent. You can do
this by choosing the creature made by the person with the earliest birthday in a calendar year.
Frequency of tt we already know is 0.24
2. Build a diploid set of chromosomes for the chosen creature.
To check your work, add the values to make sure they add up to 1: (0.26) + (0.50) + (0.24) = 1
3. Take the chromosomes through the process of meiosis to produce gametes. First, build a sister chromatid
for each chromosome in the homologous pair and attach the sister chromatids together with a centromere.
Calculating allele frequencies in the original population

Now, let’s think about recombination. Will recombination occur during meiosis? Locate the hand and beak
genes on your chromosomes. Since they are found on separate chromosome pairs, they are non-linked genes
and will segregate independently into gametes. Recombination can also occur in the form of crossover on
linked genes. The hand gene is located farthest from the centromere, so crossover is a possibility with the first
chromosome pair. But the beak gene, located close to the centromere, is an unlikely candidate for crossover. For
the purpose of this investigation, we will assume that crossover will not occur in the beak gene.
We will study the genes for hands and beak in Crazy Creatures because we know that both traits follow the law
of dominance. For the purpose of our study, let’s use new letters as symbols for each trait.
For hands, let H = paws and h = claws
For beak, let B = trumpet and b = crusher
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4. With that information, model crossover with the hand gene, then separate homologous pairs for the first
division.
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Guiding the INVESTIGATION
Guiding the INVESTIGATION
 Calculating allele frequencies in the
 Modeling reproduction on
To increase your study population, you may wish to
complete parts 1 and 2 of the investigation on the
first day and spend some time working through
Hardy-Weinberg calculations. Then, collect data from
all of your classes and give it to the students on the
second day to complete the rest of the investigation.
Larger populations in this investigation yield better
results from the activities. Work with the entire class to
complete Table 3.
This part of the investigation assumes that students
have a solid understanding of meiosis and the
recombination forces that occur during meiosis. If your
students have not been exposed to these concepts
before, be sure they complete investigations B5
through B8 prior to this investigation. If they do have
a good understanding, you may need to review the
processes of independent assortment and crossover
with them. Work with the students as a class to make
their gametes for Part 4; then they will be able to make
them on their own for Part 5, where they will have to
model the process two more times.
original population
Walnut Island
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5. Next, separate sister chromatids at the centromere to form your gametes.
6. Place your gametes into separate plastic bags, then put those bags into a paper bag. Mix them up and pull
a gamete out of the bag.
7. Place your gametes next to the gametes of the group to which you were assigned.
8. Fill in the genotype and phenotype of the offspring for Trial 1 in Table 4. Be sure to use H/h for hands
and B/b for beak instead of T/t.
9. Return the gametes to their original paper bags, mix them up, and repeat steps 6 through 8 for a total of
10 trials.
10. Fill in the last column of Table 4 with a value for fitness. The term fitness refers to the ability of an
organism to survive and reproduce. In our system, a fitness of 10 means the greatest chance of surviving
to reproduce and 1 means the smallest chance. Use the following key as your guide to assigning values:
claws and crusher beak = 10
claws and trumpet beak = 7
paws and crusher beak = 5
paws and trumpet beak = 1
C2
Modeling two more generations

In genetic variation, the alleles of organisms within a population can change through natural selection.
Natural selection is the process by which organisms with favorable adaptations survive and reproduce at
a higher rate than do those with less-favorable adaptations. Genetic variation is important to the process of
natural selection. Variations in alleles and phenotypes caused by mutations and recombination happen by
chance, but the process of natural selection is influenced by the environment. To model the influence of the
environment, follow the steps below to produce the F2 and F3 generations.
1. Choose the creature from Table 4 that has the highest fitness value.
2. Build that creature’s diploid set of chromosomes.
3. Follow steps 3 through 7 of Part 4 to produce the F1 creature’s gametes.
5. Follow steps 1 through 4 above to complete Table 6 for the F3 generation.
Table 5: F2 generation on Walnut Island
Trial
Genotype for
hands
Phenotype for
hands
1
Hh
2
Trial
Genotype for
hands
Phenotype for
hands
Genotype for beak
Phenotype for
beak
Fitnessvalue
Fitnessvalue
1
trumpet
1
2
Hh
paws
Bb
trumpet
1
hh
claws
Bb
trumpet
Bb
trumpet
1
7
3
hh
claws
BB
trumpet
Bb
trumpet
7
7
4
hh
claws
bb
crusher
10
bb
Bb
crusher
5
5
Hh
paws
bb
crusher
5
trumpet
1
6
Hh
paws
bb
crusher
BB
5
trumpet
1
7
HH
paws
BB
trumpet
Bb
1
trumpet
1
8
hh
claws
Bb
trumpet
7
bb
crusher
5
9
hh
claws
bb
crusher
10
Bb
trumpet
7
10
Hh
paws
Bb
trumpet
1
crusher
5
Genotype for beak
Phenotype for
beak
paws
BB
Hh
paws
3
hh
claws
4
Hh
paws
5
Hh
paws
6
HH
paws
7
Hh
paws
8
HH
paws
9
hh
claws
10
Hh
paws
bb
INVESTIGATION
4. Follow steps 8 through 10 of Part 4 to complete Table 5 for the F2 generation.
Table 4: F1 generation on Walnut Island
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STEM CONNECTION
A real-life Hardy-Weinberg problem Sickle-cell
anemia is a well-studied genetic disease to discuss
with your students. It also acts as a good example of
why it is advantageous to carry alleles for a genetic
disorder in a population. In sickle-cell anemia, being
normal is dominant (S) to having the disease (s).
Homozygous dominant individuals (SS) are easily
infected with the parasite carried by mosquitoes
that causes malaria. Eventually, individuals infected
with malaria will become sick and perhaps die.
Homozygous recessive individuals (ss) have red blood
cells that are “sickle” shaped and cannot properly carry
oxygen. These individuals often die at a younger age.
However, individuals who are heterozygous (Ss) have
some sickle-shaped blood cells, but not enough to
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C2 Allele Frequency and Evolution
Crazy Chromosomes
cause death. In addition, the malarial parasite cannot
survive in the blood of heterozygous individuals.
Have your students solve this problem using the
Hardy-Weinberg formulas:
Suppose 11% of a population is born with sickle-cell
anemia. What percentage of the population would be
resistant to malaria?
q 2 = 0.11 = ss
q = 0.11 = 0.33
p = 1 − 0.33 = 0.67
p 2 = (0.67 ) = 0.45 = SS
2
2 pq = 2 (0.67 )(0.33) = 0.44 = Ss
In this example, 44% of the population would be
heterozygous and resistant to malaria.
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Explore
INVESTIGATION
Explore
C2
INVESTIGATION
C2
a. How do the allele frequencies compare between the original population and the population on
Walnut Island?
Table 6: F3 generation on Walnut Island
Trial
Genotype for
hands
Phenotype for
hands
Genotype for beak
Phenotype for
beak
Fitnessvalue
1
hh
claws
Bb
trumpet
7
2
hh
claws
bb
crusher
10
3
hh
claws
bb
crusher
10
4
Hh
paws
bb
crusher
5
5
Hh
paws
Bb
trumpet
1
6
Hh
paws
Bb
trumpet
1
7
hh
claws
bb
crusher
10
8
hh
claws
Bb
trumpet
7
9
Hh
paws
bb
crusher
5
10
hh
claws
bb
crusher
10
The frequency of the recessive allele for both genes increased, while
the frequency of the dominant allele for both genes decreased.
b. Does this data provide evidence that evolution is occurring on Walnut Island? Use evidence from your
data to argue your explanation.
Yes. Since the only food source on Walnut Island is walnuts,
having claws is favorable to having paws. Also, having a crusher
beak is favorable to having a trumpet beak. Both phenotypes are
recessive and the data shows that the frequency of the recessive
allele for both genes is much higher than it was in the original
population.
c. If the dominant allele completely disappeared for both traits in the Walnut Island population, would that
be a good or a bad thing? Explain your answer.
It would be a bad thing, because genetic variation is important to
the survival of a population. If the environment changed somehow,
genetic variation would allow for other phenotypes that could be
better adaptations to the new environment.
Analyzing the data

Now, 100 years after the storm, we are studying the F3 generation on Walnut Island. Our population consists
of all of the individuals in Table 6 for the entire class. Your teacher will help you fill out columns 3 and 4
of Table 7. Using that information, calculate the allele frequencies for hands and beak in the Walnut Island
population. Then, transfer the allele frequencies for the original population from Table 3 into Table 7.
Table 7: Allele frequencies for Walnut Island
Trait
Number of
Frequency of
Frequency of recessive
homozygous recessive homozygous recessive
allele
individuals
individuals
(h or b)
40
Total population
Hands
Beak
18
16
0.45
0.40
Allele frequencies of the original population
Trait
Hands
Beak
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d. Look at the phenotypes of your original creature in Table 1. Does it have any other phenotypes that you
think would be favorable for the Walnut Island habitat? Choose one phenotype and describe why it would
be favorable. What would you expect to happen to the allele frequency for that allele if your creature
ended up on Walnut Island?
Frequency of
dominant allele
(H or B)
Recessive allele
(h or b)
Dominant allele
(H or B)
0.36
0.46
0.64
0.54
9 of 10
0.67
0.63
My creature has long arms, which would be a good adaptation for
picking walnuts off of the trees if there are none on the ground.
Long arms are dominant to short arms so I would expect the allele
frequency for T to increase on Walnut Island and the allele for t
(short legs) to decrease over time.
0.33
0.37
Allele frequencies of the Walnut Island
population
Recessive allele
Dominant allele
(h or b)
(H or B)
0.67
0.63
0.33
0.37
C2 Allele Frequency and Evolution
Crazy Chromosomes
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Crazy Chromosomes
Guiding the INVESTIGATION
 Analyzing the data
You may want to collect data from all of your classes
prior to completing Part 6, especially if you used that
larger data set earlier in the investigation. Work with
the entire class to complete the numbers in the top
rows of Table 7. Then, have students compile the
numbers in the bottom rows for comparison.
You may wish to use this opportunity to have
students create bar graphs of the allele frequencies
for comparison. This is a good visual way to compare
the data.
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Allele Frequency and Evolution
Evaluate
INVESTIGATION
C2
Notes and Reflections
Name ____________________________________________ Date ________________________
1. What is evolution?
Evolution is the change in the genetic makeup of a population
over time.
2. How are the Hardy-Weinberg formulas used by scientists to determine if evolution is occurring?
We can measure a change in the frequency of an allele between two
separate populations to determine if the frequencies have changed
between the two. If they have changed, then we can hypothesize
that evolution is occurring.
3. What is natural selection and how is it related to evolution?
Natural selection is the process by which organisms with favorable
phenotypes are able to reproduce and pass the alleles for those
phenotypes on to offspring. Natural selection is the driving force
behind evolution.
4. What is meant by the term fitness, from an evolutionary point of view?
Fitness measures the ability of an individual to survive, reproduce,
and pass its alleles on to future generations.
5. In a population of 100 Crazy Creatures, 17 individuals have the star antenna shape, which is recessive
to having a knob-shaped antenna. Calculate the allele frequency of the dominant and recessive alleles.
Then, calculate the frequencies of homozygous dominant, heterozygous, and homozygous recessive
individuals.
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C2 Allele Frequency and Evolution
Crazy Chromosomes
Question tt5 =
answer
17 ÷ 100 = 0.17
t = 0.17 = 0.41
T = p + 0.41 = 1
p = 0.59
p 2 + 2 pq + q 2 = 1
TT = p 2 = 0.592 = 0.35
Tt = 2 pq = 2(0.59)(0.41) = 0.48
tt = 0.17
WRAPPING UP
Have students reflect on what they learned from
the investigation by answering the following
question:
We know it is important to study the evolution of
genes at an individual level. Why is it also important
to study the evolution of a species at the population
level?
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