Which one of the following mutations partially or fully reverses the harmful effects of a previous mutation?

Sickle cell anemia is caused by a mutation in the hemoglobin gene known as:

What affects the rate of mutation?

How do $X$-rays induce mutations?

Frameshift mutation arises due to

The following characteristics are most suitable for which of the following processes?
$(i)$ Due to alteration of $DNA$ sequences,the genotype is changed.
$(ii)$ Changes occur in the phenotype.
$(iii)$ Variation is created in $DNA$.