Download The Sickle-Cell Mutation Enrichment LESSON 3

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Date
Class
Enrichment
LESSON 3
The Sickle-Cell Mutation
In the 1940s, doctors in Africa began to
notice that many patients who survived
malaria had a common mutation—their
hemoglobin was genetically different from
those who got the disease. This genetic
change gave the patients’ red blood cells
a sickle shape, rather than a normal disk
shape. The change was also associated with
a high risk of contracting a potentially fatal
blood disease called sickle-cell disease. In
essence, the sickle-cell mutation protected
the patient from malaria, but gave the
patient another deadly disease.
disease. But he or she does not have the
disease. In the Punnett square shown here,
normal hemoglobin has the dominant
allele S, and the sickle-cell mutation has
the recessive allele s. If both parents are
heterozygous for the sickle-cell mutation,
there is a 25 percent chance that their
offspring will have sickle-cell disease.
S
s
SS
Ss
Ss
ss
S
Natural Selection
Sickle-Cell Genotype
A person who is heterozygous for the
sickle-cell mutation is a carrier of the
s
Treatment
Sickle-shaped red blood cells cannot
easily pass through blood vessels. The result
is intense pain, fever, infections, and
weakness. Currently, no widespread cure
for sickle-cell disease exists, although some
patients have benefited from bone-marrow
transplants. Doctors usually give patients
an arsenal of different treatments to
combat the disease, such as blood
transfusions, antibiotics, and drugs that
help the red blood cells obtain oxygen.
Applying Critical-Thinking Skills
Directions: Answer each question or respond to each statement.
1. Classify the offspring of the heterozygous parents in the Punnett square as normal
hemoglobin, carrier, or sickle cell.
2. Predict What might happen to the sickle-cell mutation if malaria in Africa were
eliminated?
58
Genetics
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Scientists estimate that in some areas of
Africa, up to 40 percent of the population
carries at least one sickle-cell gene. Those
people who carry two sickle cell genes, one
from each parent, have sickle-cell disease.
The mutation is most common in those
parts of the continent that are hit hardest
by malaria—the tropical and subtropical
lowlands. The malaria pathogen does not
thrive in sickle-shaped red blood cells. So
people with the sickle-cell mutation tend
to survive, reproduce, and pass on the gene
to future generations.