Sickle-cell anemia is associated with a mutation in the gene encoding the beta subunit of hemoglobin that results in a change from glutamic acid to valine at position 6. All other amino acids are identical to a normal hemoglobin molecule. Based on the information above, which of the following mutations is the most likely cause of sickle-cell anemia?

a. A single base-pair substitution in the gene encoding the beta subunit. b. A single base-pair insertion in the gene encoding the beta subunitc. A single base-pair deletion in the gene encoding the beta subunit
d. A translocation of DNA from one chromosome to another

Explanation:

1. Sickle cell anemia is an inherited condition, disorder (disease).

2. It is a recessive condition, in which  red blood cell  become mutated and converted to a sickle like shape, and  there aren't enough healthy red blood cells to carry oxygen throughout the body.

3. This condition normally occur when, there is inheritance of two abnormal or mutated  copies of the β-globin gene, one allele from each parent.  

4. β-globin gene makes haemoglobin.

5. Sickle cell anemia results when glutamic acid being substituted by valine  at position 6 ( in short E6V substitution).

6. So, it a  single base-pair substitution in the gene encoding the beta subunit.