Sickle ­cell anemia is:

Explanation

Sickle-cell anemia is a genetic disorder affecting the structure of hemoglobin, leading to sickle-shaped red blood cells. It is caused by a substitution mutation in the DNA sequence of the beta-globin gene, where a single nucleotide change (A to T) results in the replacement of glutamic acid with valine at the sixth position of the beta-globin chain. This change causes hemoglobin molecules to stick together under low oxygen conditions, deforming red blood cells into a sickle shape. Option B is correct because it directly describes the genetic mutation responsible for the disease. Option A is incorrect because it reverses the substitution of amino acids. Option C is wrong as mature red blood cells do not have a nucleus. Option D is also incorrect because sickle-cell anemia is autosomal recessive, not dominant.