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

The obvious choice would be the hox gene.

the truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey.

the answer to your question is slate