Pleiotropy refers to the phenomenon where a single gene can affect multiple traits or phenotypes. In other words, a single gene can have multiple effects on an organism's phenotype. The correct option that represents an example of pleiotropic effect is (C) Sickle cell anemia.

Explanation:

- Sickle cell anemia is a genetic disorder that is caused by a mutation in the HBB gene, which is responsible for the production of hemoglobin. Hemoglobin is a protein that is responsible for carrying oxygen in the blood.
- The mutation in the HBB gene causes the production of abnormal hemoglobin, which can lead to the formation of sickle-shaped red blood cells. These sickle-shaped cells can cause a variety of health problems, including anemia, pain, organ damage, and stroke.
- The mutation in the HBB gene affects not only the structure of hemoglobin but also its function. This means that the mutation has multiple effects on an individual's phenotype.
- This is an example of pleiotropy because a single gene mutation affects multiple traits or phenotypes.

In contrast, the other options do not represent pleiotropic effects:

- Haemophilia (option A) is a genetic disorder that affects blood clotting. It is caused by mutations in genes that are specifically involved in the blood clotting pathway. The mutation affects only one trait or phenotype.
- Thalassemia (option B) is a genetic disorder that affects the production of hemoglobin. It is caused by mutations in genes that are specifically involved in the production of hemoglobin. The mutation affects only one trait or phenotype.
- Colour blindness (option D) is a genetic disorder that affects an individual's ability to distinguish between different colors. It is caused by mutations in genes that are specifically involved in the perception of color. The mutation affects only one trait or phenotype.

Sickle cell anemia is a pleiotropic disease because the expression of a single mutated HBB gene produces numerous consequences throughout the body. The mutated hemoglobin forms polymers and clumps together causing the deoxygenated sickle red blood cells to assume the disfigured sickle shape.