[CarolinaGuitarman]

"If a mistake occurs in one of the most significant parts of the code, which is in the genes, it can cause a disease such as sickle-cell anemia."

Not really. Sickle-cell anemia is a result of someone having two copies of the sickle hemoglobin (Hb S) allele. This allele, when carried in only one copy, infers an advantage for the person against malaria.

Sickle cell anemia is not the result of a new mutation.

[CarolinaGuitarman]

"This allele, when carried in only one copy, infers an advantage for the person against malaria."

Actually, it confers .