It depends. At one end of the spectrum, Sickle Cell Anemia is caused by a single point mutation. The sixth amino acid in the hemoglobin beta chain should be gl

It depends. At one end of the spectrum, Sickle Cell Anemia is caused by a single point mutation. The sixth amino acid in the hemoglobin beta chain should be glutamic acid, but it's been replaced by a valine. These have different charges, which is enough to warp the cell's shape and impair its function. At the the other extreme, a lot of different gene variants have been associated with autism. However, these tend not to replicate very well--the genes identified in study A don't show up in study B and vice versa, and there are other possible mechanisms, like copy number variants. CRISPR could potentially fix the former, but the latter is way beyond our current understanding.