A Lesson on the Nature of Science
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WORKSHEET
TEACHER MATERIALS
The Making of the Fittest:
Natural Selection in Humans
• His knowledge of the inheritance pattern of sickle cell disease was informed by the work of Dr. Neel,
who mathematically established that sickle cell disease is autosomal recessive.
• The knowledge of sickle cell disease began with Dr. Herrick’s diagnosis.
2. List at least two pieces of data supporting the hypothesis that there is a link between the sickle cell allele and malaria.
The parasite load was lower in the red blood cells of children with the sickle cell allele than in the red blood cells
of those without it.
There were many more children with the sickle cell allele in regions of East Africa where malaria was endemic
than in regions where it wasn’t endemic.
The frequency of children with the sickle cell allele was much lower in the arid and highland areas of East Africa
than in the coastal and wetland regions.
Other areas of the world where malaria is endemic also show an elevated frequency of the sickle cell allele.
3. Both environmental factors and genetics can account for an individual’s susceptibility to infectious disease. For the sickle
cell–malaria example, explain how genetics accounts for an individual’s susceptibility to infectious disease.
Individuals with the AA genotype (homozygous for normal hemoglobin) are susceptible to malaria infection.
Individuals with the AS genotype (heterozygous) are more resistant to malaria in part because of the inability of
the malaria parasite to reproduce successfully in these individuals. SS individuals are resistant to malaria, but
they suffer from health problems associated with having sickle cell disease. Therefore, in a malarial environment,
AS individuals experience greater fitness than either AA or SS individuals do.
4. Describe how the relationship between sickle cell disease and malaria is an example of natural selection in humans.
Natural selection is a process by which organisms most suited to their environment survive and reproduce at
higher rates. In a malarial environment, heterozygous individuals (AS) are more likely to survive and reproduce
because they are more resistant to malaria. Individuals with the AS genotype are selected for, while individuals
with the AA genotype are selected against. As a result, the frequency of the sickle cell allele is much higher in
areas where malaria is common. Although the sickle cell allele is harmful in the homozygous state (SS), it is not
removed from the population. It remains present in the heterozygous genotype from generation to generation.
In the absence of malaria, the sickle cell allele has no selective advantage.
5. In science, discoveries often lead to new questions for future investigation. With the knowledge that you have gained about
malaria and sickle cell disease through this worksheet, propose two questions that scientists could ask about either of these
diseases or the link between them.
Give students credit for any scientifically valid questions they propose. Some suggestions include the following:
• How does a fetus with sickle cell disease survive? Is there enough oxygen crossing the placenta to
prevent sickling?
• Are any animal models being used in sickle cell research?
• How does the sickle cell allele protect against malaria infection?
• Do other animals that are susceptible to malaria have sickle cell disease or something like it?
• Is there a chance that gene therapy could be used to correct the sickle cell mutation?
• Are there other genetic changes that protect against malaria, either on the hemoglobin gene or on
other genes?
AUTHOR
Mary Colvard, Cobleskill-Richmondville High School (retired), New York
FIELD TESTERS
James Coleman, Newnan High School; Cindy Gay, Steamboat Springs High School; Kimberly Hayen, Heritage High School