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CW 83: Sickle Cell Anemia: Helpful or Harmful Mutation
Problem: Is sickle cell anemia a helpful or harmful mutation?
Research: Read and mark up the text. Your goal for reading is to determine
if the sickle cell anemia gene is a harmful or helpful mutation.
A Mutation Story:
A gene known as HbS was the center of a medical and evolutionary detective story that began
in the middle 1940s in Africa. Doctors noticed that patients who had sickle cell anemia, a
Prevalentserious hereditary blood disease, were more likely to survive malaria, a disease which kills
(adjective) found
some 1.2 million people every year. What was puzzling was why sickle cell anemia was so
frequently
prevalent in some African populations.
How could a "bad" gene -- the mutation that causes the sometimes lethal sickle cell disease -also be beneficial? On the other hand, if it didn't provide some survival advantage, why had
the sickle gene persisted in such a high frequency in the populations that had it?
The sickle cell mutation is an error in the DNA code of the gene that tells the body how to
make a form of hemoglobin (Hb), the oxygen-carrying molecule in our blood. Every person
has two copies of the hemoglobin gene. Usually, both genes make a normal hemoglobin
protein. When someone inherits two mutant copies of the hemoglobin gene, the abnormal
form of the hemoglobin protein causes the red blood cells to lose oxygen and warp into a
sickle shape during periods of high activity. These sickled cells become stuck in small blood
vessels, causing a "crisis" of pain, fever, swelling, and tissue damage that can lead to death.
This is sickle cell anemia.
But it takes two copies of the mutant gene, one from each parent, to give someone the fullblown disease. Many people have just one copy, the other being normal. Those who carry the
sickle cell trait do not suffer nearly as severely from the disease.
Researchers found that the sickle cell gene is especially prevalent in areas of Africa hard-hit
by malaria. In some regions, as much as 40 percent of the population carries at least one HbS
gene.
It turns out that, in these areas, HbS carriers have been naturally selected, because the trait
confers some resistance to malaria. Their red blood cells, containing some abnormal
hemoglobin, tend to sickle when they are infected by the malaria parasite. Those infected
cells flow through the spleen, which culls them out because of their sickle shape -- and the
parasite is eliminated along with them.
Scientists believe the sickle cell gene appeared and disappeared in the population several
times, but became permanently established after a particularly vicious form of malaria
jumped from animals to humans in Asia, the Middle East, and Africa.
In areas where the sickle cell gene is common, the immunity conferred has become a
selective advantage. Unfortunately, it is also a disadvantage because the chances of being
born with sickle cell anemia are relatively high.
Beneficial(adjective) having
a good effect
Hemoglobin(noun) oxygencarrying molecule
in red blood cells
Carrier-(noun)
transmitter of
disease; infected
with a disease
without
displaying any
symptoms and
can pass it to
others
Cull- (verb) to
remove
Resistance(noun) ability to
withstand
damaging effects
Parasite-(noun)
organism living
on another that
may or may not
harm the host
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For parents who each carry the sickle cell trait, the chance that their child will also have the
trait -- and be immune to malaria -- is 50 percent. There is a 25 percent chance that the child
will have neither sickle cell anemia nor the trait which enables immunity to malaria. Finally,
the chances that their child will have two copies of the gene, and therefore sickle cell anemia,
is also 25 percent. This situation is a stark example of genetic compromise, or an
evolutionary "trade-off."
What is MALARIA?
Malaria is a mosquito-borne disease caused by a parasite. People with malaria often experience fever,
chills, and flu-like illness. Left untreated, they may develop severe complications and die. Each year 350500 million cases of malaria occur worldwide, and over one million people die, most of them young
children in Africa south of the Sahara.
This sometimes fatal disease can be prevented and cured. Bednets, insecticides, and antimalarial drugs
are effective tools to fight malaria in areas where it is transmitted. Travelers to a malaria-risk area
should avoid mosquito bites and take a preventive antimalarial drug.
Statistics about Malaria
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It is a public health problem today in more than 100 countries inhabited by some 2,400 million people -- 40
percent of the world's population.
Malaria is estimated to cause 300- 500 million clinical cases and over one million deaths each year.
Every 30 seconds, a child somewhere dies of malaria. In any given year, nearly ten percent of the global
population will suffer a case of malaria.
Most survive after an illness of 10-20 days.
Children are especially vulnerable to malaria. In Africa, where 80% of malaria cases are treated at home, the
disease kills one child in twenty before the age of five.
Pregnant women are also at high risk. They have an increased risk of disease and death, as well as adverse
impacts for their developing babies- including low birth weight, growth retardation, still births and death.
In African countries, up to 60% of hospital admissions may be for malaria; that's 6 out of 10 admissions!
Travelers to Sub-Saharan Africa have the greatest risk of both getting malaria and dying from their infection. All
travelers to any countries with malaria risk may get this potentially deadly disease, and thus taking proper
precautions is essential.
Other high-risk groups include refugees, displaced persons, or labour forces entering into endemic areas.
Malaria is transmitted in large areas of Africa, Central and South America, the island of Hispaniola (includes Haiti,
Jamaica and the Dominican Republic), Asia (including the Indian subcontinent, Southeast Asia and the Middle
East), Eastern Europe, and the South Pacific.
Research: Define mutation and describe it can be helpful and how it can be
harmful. Then write what you know about sickle cell anemia.
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Hypothesis: Based on your knowledge and understanding of sickle cell
anemia do you think the mutation that causes sickle cell is harmful or
helpful? Explain why.
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Notes from the video clip:
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Conclusion: Based on your research write a paragraph answer the question
is sickle cell anemia a helpful or harmful mutation.
First write a claim: Sickle-cell anemia is ______________. Then complete
the graphic organizer with bulleted points. Final write your paragraph.
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Sentence Topics
Details (facts from data, quotes from readings,
notes about mutations)
What is sickle-cell?
What is malaria?
What is a mutation? Is this
mutation harmful?
Evidence to support claim (from
video clip)
Evidence to support claim(from
video clip)
Concluding statement: Is sicklecell anemia a helpful or harmful
mutation?
Sickle-cell anemia is ______________.
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