Final answer:
Sickle-cell disease is caused by a point mutation in the gene coding for the beta chain of hemoglobin, leading to the production of abnormal hemoglobin and sickle-shaped red blood cells.
Step-by-step explanation:
Sickle-cell disease is a genetic condition that affects hemoglobin in red blood cells. This disease is caused by a point mutation in the DNA coding for the beta chain of hemoglobin. It leads to the production of an abnormal hemoglobin known as hemoglobin S. The mutation involves a single base change which substitutes the amino acid valine (GTG) for glutamic acid (GAG). This seemingly small change in the gene has profound effects on the form and function of hemoglobin, causing red blood cells to take on a distinctive sickle or crescent shape. These malformed cells can lead to blockages in blood flow and a lack of oxygen to parts of the body, resulting in the symptoms of anemia.
The answer to the student's question, 'Sickle-cell disease is an example of:', is a) Point mutation. This mutation affects only a single nucleotide, rather than causing a larger frameshift or altering a regulatory protein. Individuals with one copy of the mutated gene (heterozygotes) typically have sickle cell trait and are usually not affected, while homozygotes, who have two copies of the mutated gene, tend to suffer from the more severe symptoms of sickle-cell disease.