Looking at all the pokedex entries, they are both genderless. Mewtwo was the only successful clone of mew and judging by ditto's appearance, it looks man made. "Mew is said to contain the DNA of every single Pokemon, while Ditto can essentially breed with every single Pokémon" (except legendaries).
It seems that ditto was a failed clone.
EDIT: Of course there is no way we can be 100% certain. Unless the anime reveals so, or any other source from gamefreak, etc.
EDIT2: Both Mew and Ditto have base stats that are the same in every category. Mew has 100 base in each stat, while Ditto has 48 base. The failed cloning process may have resulted in such diminished stats