If the gene sequence on a chromosome changes from $abcdefg$ to $abcfedg$,what is this phenomenon called?

The idea of mutations was brought forth by

Agents that induce mutations are called ....

$A$ mutant microorganism is unable to synthesize a component required for its growth,but it is able to grow if that component is provided. This is called a/an......

What is the name of the disease that serves as a classic example of point mutation?

Which disease serves as the most classic example of a frameshift mutation?