IGCSE Biology Paper-4: Specimen Questions with Answers 171 - 172 of 279

Passage

Sickle-cell anemia is an inherited disease.

Show is a photomicrograph of some blood cells from a person who has sickle-cell anemia.

Sickle-Cell Anemia

Question 171 (1 of 4 Based on Passage)

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Sickle-cell anemia is most common in areas of the world where the infectious disease malaria is found.

Some species of the genus Plasmodium cause malaria in humans.

(ii) The distribution of sickle-cell anemia is the result of natural selection.

Explain the distribution of the sickle-cell allele in human populations.

Explanation

Natural Selection Can Keep a Harmful Allele in a Gene Pool

The allele (S) for sickle-cell anemia is a harmful autosomal recessive. It is caused by a mutation in the normal allele (A) for hemoglobin (a protein on red blood cells) . Malaria is a deadly tropical disease. It is common in many African populations. Heterozygotes (AS) with the sickle-cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool. The sickle-cell example shows that fitness depends on phenotypes. It also shows that fitness may depend on the environment.

Image Show of the Sperm

Question 172 (2 of 4 Based on Passage)

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Explain how red blood cells become sickle-shaped.

Explanation

Sickle cell anemia is an inherited disease that affects red blood cells with an abnormal version of hemoglobin, the protein that carries oxygen throughout the body. The altered hemoglobin is known as hemoglobin S, or sickle hemoglobin, because it causes normally oval-shaped red blood cells to assume a sickle shape.

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