The Making of the Fittest: LESSON

Natural Selection and Adaptation STUDENT HANDOUT

MOLECULAR GENETICS OF COLOR MUTATIONS IN ROCK POCKET MICE

INTRODUCTION
THE ROCK POCKET MOUSE
The rock pocket mouse, Chaetodipus intermedius, is a small, nocturnal animal found in the deserts of the southwestern
United States. Because most rock pocket mice have a sandy, light-colored coat, they are able to blend in with the light
color of the desert rocks and sand that they live on. But populations of primarily dark-colored rock pocket mice have
been found living in areas where the ground is covered in a dark rock called basalt, which was caused by geologic lava
flows thousands of years ago. Scientists have collected data from a population of primarily dark-colored mice living in an
area of basalt in Arizona’s Pinacate lava flow, as well as from a nearby light-colored population. Researchers analyzed the
data from these two populations to search for the genetic mutation responsible for the dark coat color. Through their
analyses, they discovered a mutation in the Mc1r gene that is involved in coat-color determination.
THE MC1R GENE
The coat color of rock pocket mice is primarily determined by two pigments: eumelanin, which is dark colored, and
pheomelanin, which is light colored. The synthesis of these pigments is controlled by the products of several genes,
including the Mc1r gene. This gene encodes a protein called melanocortin 1 receptor (MC1R). This receptor is found
embedded in the membrane of melanocytes, which are cells specialized for pigment production. The melanocytes of
wild-type (nonmutant) mice produce more pheomelanin than eumelanin. The result is a sandy-colored mouse. The
mutated version of the Mc1r gene, however, triggers melanocytes to increase the production of eumelanin, resulting in
the dark coat-color phenotype.
GENE MUTATION
A gene mutation is any change in the DNA sequence of a gene. Gene mutations can change the structure of the resulting
protein. A change in protein structure can change, negate, or have no effect on function. There are several types of
mutations, and several results that mutations can have on the amino acid sequences of proteins.
Types of Mutations
• Substitution mutation: The replacement of one nucleotide of DNA for another. Mutations that affect a single
nucleotide are called point mutations.
• Insertion mutation: The addition of one or more nucleotides to the DNA gene sequence. The insertion of nucleotides
can result in frameshift mutations.
• Deletion mutation: The loss of one or more nucleotides from the DNA gene sequence. The deletion of nucleotides can
result in frameshift mutations.
Potential Results a Gene Mutation Has on a Protein
• Silent mutation: This mutation does not cause a change in the amino acid sequence of the protein; therefore, there is
no change in the resulting protein.
• Missense mutation: This mutation causes an amino acid in the sequence to be changed to another amino acid. This
type of mutation causes a change in the primary structure of the protein (the linear sequence of amino acids), which
typically results in a change in the three-dimensional conformation of the protein.
• Nonsense mutation: This mutation causes the protein to be truncated (cut short) due to the incorporation of a “stop”
signal into the DNA sequence. This results in translation being stopped before the amino acid sequence of the protein
is completed.

MATERIALS
genetic code chart (see page 7 of this handout or a biology textbook)
blue, red, and green colored pencils

Molecular Genetics of Color Mutations in Rock Pocket Mice Published July 2012
Revised October 2013
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 The Making of the Fittest: LESSON
Natural Selection and Adaptation STUDENT HANDOUT

PROCEDURE
1. Watch the short film The Making of the Fittest: Natural Selection and Adaptation.
2. Using the DNA nucleotide sequence in the gene tables (page 3), determine the complementary messenger RNA
(mRNA) sequence for the portion of the Mc1r gene provided. (Note: You are only transcribing a small portion of the DNA
sequence for this protein. The actual gene contains 951 base pairs.) The numbers above some of the columns indicate
amino acid positions in the protein sequence. Also indicated is whether the sequence codes for an extracellular,
intracellular, or transmembrane part of the protein.
3. Using the mRNA sequence determined in Step 2, determine the resulting amino acid sequence of the MC1R protein.
(Note: This is only a portion of the 317 amino acids in the entire protein. The numbers above some of the columns in the
tables indicate amino acid positions in the protein sequence.) You may use the genetic code chart provided in your
textbook or the one on page 7 of this handout.
4. There are five mutations in the dark-color Mc1r mutant gene. Compare the DNA sequence of the wild-type Mc1r gene
with the DNA sequence of the mutant Mc1r gene. Indicate the locations of the five mutations by circling the five single
DNA nucleotides that are mutated in the mutant Mc1r gene table.
5. From the introduction, determine whether each of these mutations is a silent, missense, or nonsense mutation.
a. Using the mutant Mc1r gene data, shade in the columns (including DNA, mRNA, and amino acid) in the mutant
table that contain a silent mutation. Use a blue colored pencil to do this.
b. Likewise, use a red colored pencil to shade in the columns that contain a missense mutation.
c. Shade any columns that contain nonsense mutations by using a green colored pencil.
6. Answer the questions following the gene tables.

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GENE TABLES
WILD-TYPE MC1R GENE (LIGHT-COLORED COAT PHENOTYPE)
015 024
DNA TTG AGG TGG GCG TGT CCG CAA GGA GTG GAG
mRNA

Extracellular Domain I
Amino
Acid
MUTANT MC1R GENE (DARK-COLORED COAT PHENOTYPE)
015 024
DNA TTG AGG TGG ACG TGT CCG CAA GGA GTG GAG
mRNA
Amino
Acid

WILD-TYPE MC1R GENE (LIGHT-COLORED COAT PHENOTYPE)

105 114
DNA CGG GAC CGG TGG GCC CAC TGA CAC CAT GTC
mRNA

Extracellular Domain III

Amino
Acid
MUTANT MC1R GENE (DARK-COLORED COAT PHENOTYPE)
105 114
DNA CGG GAC CGG TGG ACC CAC TGA CAC CAT GTC
mRNA
Amino
Acid

WILD-TYPE MC1R GENE (LIGHT-COLORED COAT PHENOTYPE)

154 163
DNA TCA TAA CAC TGT GAC GGG GCC CGA GCC ACC
mRNA
Intracellular Domain I

Amino
Acid
MUTANT MC1R GENE (DARK-COLORED COAT PHENOTYPE)
154 163
DNA TCA TAA CAC TGT GAC GGG ACC CGA GCC ACC
mRNA
Amino
Acid

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WILD-TYPE MC1R GENE (LIGHT-COLORED COAT PHENOTYPE)

208 212
DNA CAC GTG TAC GAA CGT
mRNA

Transmembrane V
Amino
Acid
MUTANT MC1R GENE (DARK-COLORED COAT PHENOTYPE)
208 212
DNA CAC GTG TAC GAG CGT
mRNA
Amino
Acid

WILD-TYPE MC1R GENE (LIGHT-COLORED COAT PHENOTYPE)

230 239
DNA GAA CAG GTG GTT CCA AAG GCT GAG TTT CCG
mRNA

Intracellular Domain III

Amino
Acid
MUTANT MC1R GENE (DARK-COLORED COAT PHENOTYPE)
230 239
DNA GAA CAG GTG GTG CCA AAG GCT GAG TTT CCG
mRNA
Amino
Acid

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 The Making of the Fittest: LESSON
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QUESTIONS
1. Using the amino acid numbers provided above the first and last column of each table, list the locations of the five
amino acids that contain a mutation.
_______ _______ _______ _______ _______
2. Of the five mutations you identified in the Mc1r gene, how many are the following:
_____ substitutions _____ insertions _____ deletions (Enter a number on each line.)
3. Of the five mutations you identified in the Mc1r gene, how many are the following:
_____ silent _____missense _____ nonsense (Enter a number on each line.)
4.
a. Which four amino acid locations (see Question 1 above) contain the missense mutations?
_______, _______, _______, _______
b. Explain the link between DNA sequence and protein structure and function.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
5. Using the information on mutations in the introduction and your knowledge of proteins, develop a hypothesis to
explain how the changes in the MC1R protein’s amino acid sequence might affect its function.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
6. Many proteins, including MC1R, contain several structural domains that can fold and function independently from the
rest. The domain names were provided for each portion of DNA sequence you translated earlier. Answer the following
questions.
a. Where is the MC1R protein found, and what is its function? Be specific.
____________________________________________________________________________________
____________________________________________________________________________________
b. Which protein domains contain the four Mc1r missense mutations? (Refer to the gene tables you completed
earlier.)
____________________________________________________________________________________
____________________________________________________________________________________
c. Define “extracellular.”
____________________________________________________________________________________
d. Define “intracellular.”
____________________________________________________________________________________
e. Why is it significant that the four missense mutations are found in the extracellular and intracellular domains of the
protein? Explain your answer. (Hint: Think about MC1R’s function.)
____________________________________________________________________________________

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 The Making of the Fittest: LESSON
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____________________________________________________________________________________
7. Using the information on the Mc1r gene in the introduction and your knowledge of proteins, develop a hypothesis to
explain how the change in MC1R protein function might directly affect a rock pocket mouse’s coat color. Be specific and
consider both the light-colored and dark-colored phenotypes.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
8. Explain why the mutation at amino acid location 211 is not as significant as the other four mutations.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
9. Mutations are a source of genetic variation. In the film, Dr. Sean Carroll says that mutations occur randomly. What does
this mean?
____________________________________________________________________________________
____________________________________________________________________________________
10. It is a common misconception that “all mutations are bad.” Use the example of rock pocket mice to explain why this
statement is not true. In your answer, explain how the dark coat-color mutation can be an advantage to some mice and a
disadvantage to others.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
11. Use your understanding of evolution and the information in the film to explain how the dark-colored mutation came
to be so common in some populations of rock pocket mice. Be specific.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________

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 The Making of the Fittest: LESSON
Natural Selection and Adaptation STUDENT HANDOUT

GENETIC CODE CHART

Below is the standard genetic code: mRNA  amino acid. The inner circle represents the first letter of the codon, and the
remaining letters follow in the second and third rings. Find the corresponding amino acid in the outer circle.

AUTHOR
Ann Brokaw, Rocky River High School, Ohio

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