Turn off a gene, grow a second head

It is that simple, well it is if the patient is a jellyfish.

If a cer­tain Cnox gene is shut down, they found they could gen­er­ate hy­dro­zo­an jel­ly­fish of the spe­cies Eleu­the­ria di­cho­to­ma with two heads, that both fully func­tion and take in food. De­ac­tiva­t­ion of a dif­fer­ent gene led to even more heads, they added.

The rar­ity of such beasts in na­ture may be be­cause mul­ti­ple heads of­fer lit­tle or no ad­van­tage, Ja­kob and Schier­wa­ter said.

So, if I am correct in thinking that during the last global cataclysm, cosmic ray bombardment caused enough DNA mutations to create new species, the many-headed Hydra of ancient greek myths could have been a real species, for a while. So too could have the Cerberus once existed – a monstrous three-headed dog that guards the gate to Hades... (more multi-headed creatures).

Obviously it wouldn't have lasted too many generations, but possibly long enough to be recalled in myths.