A phenotype refers to the observable physical or biochemical characteristics of an organism, resulting from the interaction of its genotype (genetic makeup) and the environment. In this question, we are asked to identify a phenotype that arises specifically from a genetic mutation.

- Sickle-cell anemia is a classic example of a disease caused by a specific genetic mutation in the hemoglobin-beta gene. This mutation results in abnormal hemoglobin molecules that cause red blood cells to assume a sickle or crescent shape, leading to clinical symptoms such as anemia, pain crises, and organ damage. Thus, it is a phenotype directly resulting from a mutation.

- Option 2, Point mutation, refers to the change of a single nucleotide base in DNA. While this is the type of mutation that causes sickle-cell anemia, it is a genetic change, not a phenotype by itself.

- Option 3, Ribosomal RNA, is a type of RNA involved in protein synthesis, not a phenotype or a mutation.

- Option 4, Mitochondrial DNA, refers to DNA located in the mitochondria. It can carry mutations that cause disease, but mitochondrial DNA itself is not a phenotype.

In summary, sickle-cell anemia represents a clear example of a phenotype caused by a genetic mutation and is the best answer to this question.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 3: Genetic Disorders / Page 67