there is considerable variation between the genomes of any two individuals,
but only a small amount of that variation appears to have any significant
biological impact, that is, produces differences in function. The Human
Genome Project, completed in 2003, illuminated the extent of human
genetic variation as well as the variations that have biological significance.
This lesson uses an examination of variation in a 1,691-base segment of
the beta globin gene to help students consider the extent of human genetic
variation at the molecular level and the relationships between genetic
variation and disease and between genetic variation and evolution.
In Advance
Web-Based Activities
Day 1, Step 12.
Day 2, Step 4.
Materials and Preparation
Photocopies and Transparencies
Equipment and Materials
• 1 copy of Master 2.1 for each student
• (Optional) Computers
• 1 copy of Master 2.2 for each student
with Internet access
• 1 copy of Master 2.3 for each student
• 1 copy of Master 2.4 for each student
• 1 copy of Master 2.5 for each student*
• 1 copy of Master 2.6 for each group
*Needed only by classes without Internet access.
Day 1, Step 12, describes an optional laboratory exercise that you may wish
to conduct to enrich your students’ understanding of molecular variation
and the methods by which it can be identified and studied. Information
about the materials you’d need is on page 77.
Follow the instructions on page 77 to get to the Web site students will use
to view the video documentary (Step 12). If you do not have Internet access,
you can use the print-based alternative (also discussed on page 77).
72
DAY 1
Procedure
1. Introduce the lesson by asking students to identify the ultimate source
of the variation they investigated in Lesson 1.
Students should recognize that the ultimate source of genetic variation
is differences in DNA sequences.
2. Explain that in this lesson, students will investigate human genetic
variation at the molecular level and examine the impact of that
variation on biological function. Give one copy of Master 2.1, How
Much Variation? Beta Globin Gene—Person A, and Master 2.2, How
Much Variation? Beta Globin Gene—Person B, to each student. Explain that the sequences on these pages come from the beta globin gene in
two different people.
Hemoglobin, the oxygen carrier in blood, is composed of four
polypeptide chains, two alpha polypeptide chains and two beta
polypeptide chains. The beta globin gene encodes the amino acid
sequence for the beta chain. Person A and person B each show 1,691
nucleotides from the “sense” strand of the gene (that is, the strand
that does not serve as the template for transcription and thus has
the same base sequence as the messenger RNA, with Ts substituted
for Us). Both the sense strand of DNA and the messenger RNA are
complementary to the DNA strand that serves as the template for
transcription. We recommend that you remind students that DNA is
double-stranded, even though only one strand is shown in Masters 2.1
and 2.2. Explain that geneticists use shortcuts like this because, given
the sequence of one DNA strand, they can infer the sequence of the
complementary strand.
The beta globin gene is one of the smallest human genes that encode
a protein; the entire gene has only about 1,700 nucleotide pairs and
includes just two introns. The sequences on Masters 2.1 and 2.2 do not
show the gene’s promoter regions but begin with the first sequences
that are translated.
3. Ask students to read the paragraph at the top of each page and then
estimate the total number of bases on each page. Direct students to
write their estimate in the space provided on the masters.
The total number of bases on each page is 1,691. Students will need this
number to complete their calculations in Step 6.
73
Student Lesson 2
Human Genetic Variation
4. Remind the students that the sequences on the masters come from the
beta globin gene in two different people. Ask students what they notice
when they compare the sequence from person A with the sequence
from person B.
Students should notice that most of the sequence appears to be exactly
the same in both people. They should also notice that the bases that are
in bold are different. If necessary, point out that these bases are at the
same positions in each gene (that is, be sure that students realize that
only these two bases, located in these specific positions, are different
in the sequences from person A and person B).
5. Point out that this sequence is only 1,691 bases long and the complete
human genome is about 3 billion bases long. Ask the students how
they might use the sequences from person A and person B and the
total size of the human genome to estimate the extent of variation
(the number of bases that differ) between the two people B. Ask as
well what assumption they would be making as they arrived at
their estimate.
Students could estimate the extent of variation across the entire genome
by calculating the percentage of difference between the two sequences
shown for person A and person B, and then multiplying this percentage
by 3 billion (the approximate number of bases in the human genome).
This estimate assumes that the sequence shown displays a typical
amount of variation.
6. Give one copy of Master 2.3, How Much Variation? Doing the Math, to each student and direct the students to use the master as a guide to
estimate this value.
If your students need help completing this estimate, suggest that they
first try the example at the bottom of the master.
The proportion of sequence difference between person A and person
B is 2/1,691 = 0.001 (rounded off). To make this more concrete for
your students, note that this means that about 1 base in every 1,000 is
different. The percentage difference is 0.001 × 100 = 0.1 percent.
The total number of base differences would be 3,000,000,000 × 0.001
= 3,000,000 or, in scientific notation, 3×109×10–3 = 3×106. That is,
we could expect to find 3 million base differences in DNA sequence
between any two people.
Note that the actual number of base differences between two people is
likely somewhat higher than this because this estimate, based as it is
on the approximate size of the human genome (one copy of each of the
autosomes, plus the X, Y, and mitochondrial chromosomes), does not
take into consideration the fact that humans are diploid.
74
7. Ask students what their estimates indicate about the extent of human
genetic variation at the molecular level.
Students should recognize that at the molecular level, humans are far
more alike (about 99.9 percent of the bases are the same) than they are
different (only about 0.1 percent of the bases are different). Students
should also realize, however, that even a small percentage difference
can represent a very large actual number of differences in something
as large as the human genome.
If students have difficulty reaching these conclusions, help them by
asking questions such as, “Based on this comparison, do you think that
at the molecular level, people are more alike than they are different or
vice versa?” and “How can a difference of only 0.1 percent (1 in 1,000)
result in such a large number of differences (3 million differences)?”
8. Explain that the rest of the lesson focuses on this 0.1 percent
difference between people. Ask students questions such as, “Do you
think these differences matter? What effect do you think they have?
What might affect how much a specific difference matters?”
These questions focus students’ attention on the significance of the
differences, instead of the number of differences. Remind students of
the differences among people that they observed in Lesson 1 and point
out that most of these differences have their basis in a difference in
the DNA sequence of particular genes (pierced versus nonpierced body
parts probably do not). To help them understand the magnitude of the
number of differences between their DNA and that of another person,
ask students whether they think there are 3 million differences in
appearance and biological functions between themselves and the
person sitting next to them (discussed in Step 10, below).
9. Explain that studying the beta globin gene more closely will help
students begin to answer these questions for themselves. Have
students examine the sequences on Master 2.1, Beta Globin Gene—
Person A, and Master 2.2, Beta Globin Gene—Person B, again. Explain that the regions that show bases grouped in triplets are from the
coding regions (exons) of the gene, while the other regions are from
the noncoding regions (introns). Then, ask students which of the
two base differences in bold is most likely to matter, and why.
Most eukaryotic genes are composed of both coding and noncoding
regions, which are transcribed into an initial messenger RNA. The
noncoding introns are then spliced out of the RNA; other processing
steps ultimately result in the mature messenger RNA that is translated
into protein. Students should realize that the second base difference
occurs in a noncoding region of the gene and is unlikely to have an
impact on individuals. The first difference occurs in a coding region
and is more likely to matter.
75
Student Lesson 2
Human Genetic Variation
10. Explain that although 3 million base differences sounds like a lot,
most of these differences have no significant impact on individuals,
either because they occur in a noncoding region or for another reason.
Point out that most of these 3 million differences can only be detected
A major concept
by examining the DNA sequence.
that students should
understand from Day 1
Students should now understand that while some base differences occur
of the lesson is that
in coding regions and may result in an altered amino acid sequence
in the protein coded for by a gene, others occur in noncoding regions
most genetic differences
where they likely have no impact. Point out that only a small percentage
do not affect how
of the DNA sequences in the human genome are coding sequences.
individuals function.
Furthermore, only a small percentage of the noncoding DNA sequences
are regulatory sequences such as promoters or enhancers that can
influence the amount of gene product that results from a given gene.
The remaining DNA sequences (the majority of the total DNA sequences
in the genome) have no known function. Most of the variations in DNA
sequence occur in these latter sequences and have no detectable impact.
Note: You may wish to clarify for students the reason that most
molecular variation occurs in noncoding regions. It is true that there
are more noncoding than coding regions. However, the fundamental
biological reason for the increased variability of noncoding regions is
that there is no selective pressure exerted on changes in them. Point
out that some differences that occur in noncoding regions do have an
impact. For example, several mutations within introns in the beta globin
gene cause incorrect splicing of the messenger RNA, and as a result,
several codons may be inserted into or omitted from the sequence,
leading to nonfunctional beta globin polypeptides.
If you wish to offer your students a more sophisticated understanding
of why most DNA sequence differences have no impact, extend the
discussion to include the following ideas. Even many of the differences
that occur in coding regions have no impact. Only those differences
that result in a change in amino acid sequence in a critical region of the
protein (one that affects the function of the protein) or that result in a
premature stop codon in the RNA (and thus a truncated protein) have a
significant impact on the individual carrying that variation. As students
will see in Day 2, those few differences that do affect individuals often
have devastating consequences.
11. Point out the codon in which the first difference between the
two sequences occurs and tell students that person A has normal
hemoglobin, while person B has abnormal hemoglobin that is
associated with sickle cell disease. Explain that the single base
difference in this codon determines whether a person has normal
hemoglobin or sickle hemoglobin.
If you wish, ask students to identify the actual amino acid difference
between these two types of hemoglobin, based on the difference in the
DNA sequence of the codon you identified. This is an opportunity for
76
students to review the translation process and the genetic code. Remind
them that the sequence they have is the same as the messenger RNA
sequence, except it has Ts where the RNA would have Us. Normal
hemoglobin has glutamic acid (RNA codon GAG) in the position where
sickle cell hemoglobin has valine (RNA codon GUG).
12.
Step 12 for classes with access to the Internet: Tell students
that in the next part of the lesson, they will consider the
consequences of the genetic variation that results in sickle
cell disease. Hand out Master 2.4, Exploring Sickle Cell
Disease, and direct students to organize into small groups. Have
students view the documentary “What Is Sickle Cell Disease?” on
the student section of the Human Genetic Variation Web site
( http://science.education.nih.gov/supplements/genetic/student; click
on “The Meaning of Genetic Variation”) and begin working on the
questions.
Step 12 for classes using the print version: Tell students
that in the next part of the lesson, they will consider
the consequences of the genetic variation that results in
sickle cell disease. Hand out one copy each of Masters 2.4,
Exploring Sickle Cell Disease, and 2.5, Reference Database: Sickle Cell
Disease, and direct students to organize into small groups and begin
working on the questions.
Give the students about 30 minutes to complete their research. Notice
that most of the information they need is located in the Sickle Cell
Database on the Web site and, for classes using the print version, on
Master 2.5. If the students’ textbook has an adequate description of
sickle cell disease, you may wish to assign certain questions on
Master 2.4 for them to complete at home with their textbook.
13. When students reach Question 2 on Master 2.4, they should explain
how they intend to test the Lindsey twins. Give the group a copy
of the test results (Master 2.6, Results of the Lindsey Test) after the
students correctly explain the test they would have conducted.
Optional alternative to Master 2.6: Instead of giving students the
results of the test they propose in Question 2 on Master 2.4, you may
want them to complete the relevant laboratory activity themselves.
Kits are available that you can adapt to this purpose. Some use proteins
that represent hemoglobin from normal, sickle cell, and sickle cell trait
(heterozygous) individuals. The test results on Master 2.6 are based
on DNA from such individuals. If you use a kit, be sure to make this
distinction clear to students. If you plan to have your students complete
the lab, schedule an additional half to whole class period for the
activity.
77
Student Lesson 2
Human Genetic Variation
DAY 2
1. Open the second half of the lesson by directing students to meet in
their groups to complete or review their answers to the questions on
Master 2.4, Exploring Sickle Cell Disease. After they have completed
their work, convene a class discussion in which students can share
their answers to the questions.
Question 1a. What are the primary symptoms of sickle cell disease?
What happens in a person’s body to cause these symptoms?
People with sickle cell disease periodically experience symptoms that
include severe pain and fever. The symptoms occur when the sickle
hemoglobin (Hb S) inside red blood cells forms long crystals under
conditions of low oxygen concentration. The red blood cells elongate and
assume a “sickle” shape. The crystallized hemoglobin damages the cell
membranes, causing them to burst easily. The misshapen cells also clog
blood vessels. The result is the destruction of many red blood cells within
a few hours and a disruption of oxygen transport that can lead to death.
Question 1b. How is Hb S (sickle hemoglobin) different from Hb A
(normal hemoglobin)?
Sickle hemoglobin (Hb S) has the amino acid valine in the position
where normal hemoglobin (Hb A) has the amino acid glutamic acid.
Question 1c. How can this difference in hemoglobin be detected in
the laboratory?
Because of the difference in the amino acid sequence of Hb A and Hb S,
the two forms of hemoglobin have different charges. The two forms can
be separated using electrophoresis because Hb S moves more slowly in
an electric field than Hb A.
Question 1d. What does this difference in hemoglobin tell you about the
DNA of people whose cells make Hb S as compared with people whose
cells make Hb A?
The sequence of DNA that codes for hemoglobin in people whose cells
make Hb S must be different from the sequence of DNA that codes for
hemoglobin in people whose cells make Hb A. The allele that codes for
Hb A has the nucleotide A at a place where the allele that codes for Hb S
has the nucleotide T.
Question 1e. What is the difference between sickle cell disease and sickle
cell trait? Demonstrate in your answer that you understand how sickle
cell disease is inherited.
People who have sickle cell disease have inherited two alleles for sickle
cell hemoglobin, one from each of their parents. They are homozygous
for the sickle cell hemoglobin allele. People who have sickle cell trait
78
have inherited one allele for sickle cell hemoglobin from one parent
and one allele for normal hemoglobin from the other parent. They are
heterozygous and usually have no symptoms.
Question 2. Use what you learned about sickle cell disease and trait to
propose a way to determine whether Ms. Lindsey’s twins have sickle cell
trait. Explain your procedure to your teacher, then use the information
provided on the handout your teacher will give you to determine the
results of the test.
Highlight the contribution
of basic science to the
Students should explain the following procedure: Collect DNA from
improvement of personal
Jason and from Sondra and treat it for analysis of alleles of the beta
and public health by
globin gene. Use gel electrophoresis to visualize the alleles present
asking your students
in each twin’s DNA. “Standards,” or controls, of DNA from people
with the normal ( Hb A) and sickle ( Hb S) alleles should be included
whether an early-20th-
for comparison. (Unless you have previously discussed restriction
century physician would
enzymes and RFLP analysis, you probably do not want to introduce
have answered Ms.
these concepts here. Instead, just explain to students that DNA isolated
Lindsey’s question in this
from individuals can be treated in ways that make different alleles show
manner. The answer, of
different gel electrophoresis patterns.)
course, is no. The fi rst
If a twin has normal hemoglobin, his or her DNA will migrate on
observation of sickle-
the gel in the same pattern as the DNA standard for the Hb A allele.
shaped cells was made in
If a twin has sickle cell disease, his or her hemoglobin will migrate
1910, but the molecular
in the pattern of the DNA standard for the Hb S allele. If a twin is
basis of the disease was
heterozygous (has sickle cell trait), his or her DNA will contain two
not worked out until
different alleles for the hemoglobin gene and show both the pattern
1949. You may also note
of the