IGCSE Biology Paper-4: Specimen Questions with Answers 173 - 174 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 173 (3 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.

(i) Define the term species.

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Explanation

It is the basic unit of classification. According to ‘Biological concept of species’ , the species can be defined as a group of natural populations whose members can interbreed among themselves and are reproductively isolated from other such groups.

Question 174 (4 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.

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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