Answer:
A. are caused by genes for defective proteins
Step-by-step explanation:
Sickle cell anemia occurs when the gene coding for the beta chain of the hemoglobin has a substitution mutation. The mutant gene code for a faulty protein. The mutated hemoglobin forms aggregates and imparts sickle shape to the red blood cells carrying them. Similarly, hemophilia is caused due to mutation in the gene coding for Factor VIII. This protein is involved in blood clotting. The affected person has a deficiency of Factor VIII and experience severe blood loss even in case of a small injury.