11. if a single mutation turns off the growth of some pairs of legs within an organism, what's most likely affected?

The answer is hox gene.

Hox genes are responsible for an embryo's body plan. They determine different body structures - legs, vertebrae, antennae, wings, depending on groups. Mutation in hox genes is serious mutation since it affects the early stages of embryo and consequently, normal development. If a mutation occurs in the hox gene for legs growth, the legs will not develop.