Final answer:
Sickle-cell anemia is caused by the recessive allele Hbs, which leads to a mutation in the beta chain of hemoglobin. When homozygous, it causes sickling of red blood cells, disrupting normal blood flow and oxygen delivery, whereas heterozygotes may exhibit resistance to malaria without severe disease symptoms.
Step-by-step explanation:
The Genetics of Sickle-Cell Anemia
The allele Hbs that codes for sickle-cell hemoglobin results in sickle-cell anemia when present in the homozygous recessive form (ss). A single amino acid substitution in the beta chain of hemoglobin—glutamic acid being replaced by valine—causes red blood cells to deform and assume a 'sickle' shape. This affects the cells' ability to efficiently carry oxygen, leading to symptoms associated with sickle-cell anemia. Heterozygotes (carry one Hbs allele and one normal allele) often show no symptoms and are said to have the sickle-cell trait, which provides some resistance to malaria—a discovery noted by J.B.S. Haldane.
Individuals homozygous for the Hbs allele suffer from more frequent and severe episodes due to the sickling of their red blood cells, which disrupts normal blood flow and oxygen delivery. This mutation originated in Africa and has a higher prevalence in regions where malaria is or was endemic. Stress factors such as infection or dehydration can exacerbate the sickling process in these patients.