What cause sickle-cell disease?

Answer 1

It's caused by a mutation in the hemoglobin gene (a weird change that's basically in the red blood cell).

It has to come from two parents who both have a trait of sickle cell. Not all of their kids will have it though. Some kids might have the trait, while some will actually have sickle cell anemia, and some might have no trait of it at all. It's a strange, strange mutation.
Sickle cell disease is caused by a point mutation in the gene that codes for hemoglobin which is the primary protein found in the red blood cells. Basically, in the sequence of that gene, one letter changes and causes the entire structure of the hemoglobin molecule to change. The affinity of oxygen to hemoglobin decreases so the mutated red blood cell ends up carrying less oxygen compared to a normal red blood cell.

Due to this change, the red blood cell looks like a sickle so its is called a Sickle Cell Disease.