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Difficulty: MediumCell Biology, Genetics, and Evolution

In the study of molecular genetics and evolutionary biology, sickle-cell anemia serves as a classic model of interaction between genetic mutations and natural selection. Which of the following statements correctly describes the genetic basis of sickle-cell anemia and its evolutionary significance in human populations?

  1. A
    It is caused by a chromosomal trisomy that confers broad immunity against viral and bacterial infections in tropical environments.
  2. It is caused by a point mutation in the beta-globin gene, and heterozygous individuals exhibit higher survival rates in malaria-endemic regions due to balancing selection.Answer
  3. C
    It is caused by epigenetic silencing of alpha-globin genes that prevents mosquito vectors from transmitting parasitic filarial worms.
  4. D
    It is caused by a frameshift mutation in mitochondrial DNA that halts red blood cell formation, preventing dengue virus transmission.

Answer

It is caused by a point mutation in the beta-globin gene, and heterozygous individuals exhibit higher survival rates in malaria-endemic regions due to balancing selection.
The choice stating that sickle-cell anemia is caused by a point mutation in the beta-globin gene and maintained by balancing selection due to malaria resistance is correct. A point mutation at the sixth codon of the beta-globin gene (HBBHBB) substitutes glutamic acid with valine. In malaria-endemic geographical zones, heterozygous individuals (HbA/HbSHb^A/Hb^S) possess a distinct survival advantage against severe *Plasmodium falciparum* malaria while avoiding the fatal complications of homozygous sickle-cell anemia (HbS/HbSHb^S/Hb^S), exemplifying heterozygote advantage under balancing natural selection.

Step-by-Step Solution

1
Identify the molecular and genetic basis of sickle-cell anemia
Sickle-cell disease is caused by a single nucleotide substitution (point mutation/transversion of A to T) in the sixth codon of the beta-globin (HBBHBB) gene on chromosome 11, replacing hydrophilic glutamic acid with hydrophobic valine (GAG→GTGGAG \rightarrow GTG).
Establishes that the condition is a monogenic point mutation rather than a chromosomal anomaly or mitochondrial inheritance.
2
Analyze the physiological phenotype and evolutionary pressure in malaria-endemic areas
Heterozygous carriers (HbA/HbSHb^A/Hb^S) produce both normal and abnormal hemoglobin, causing red blood cells infected with *Plasmodium falciparum* to sickle prematurely and be cleared by the spleen before the parasite completes its intraerythrocytic cycle.
Identifies the specific biological mechanism granting resistance to severe falciparum malaria without causing full sickle-cell disease.
3
Apply the principles of population genetics and natural selection
Heterozygote advantage (overdominance) leads to balancing selection, which actively maintains both the normal HbAHb^A and mutant HbSHb^S alleles in human populations where malaria is endemic.
Connects molecular genetics to evolutionary mechanisms.

Key Concept

Molecular point mutation in hemoglobin beta chain and balancing selection via heterozygote advantage against falciparum malaria
Estimated Time:1m 0s
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Topics

Cell Biology, Genetics, and Evolution

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