XD LOL That's too funny...Mew is soo cute when she tries to be helpful to her son. (Play Fire Red or Leaf Green. At the Pokemon Mansion, the doctor's diaries inform you that Mew gave birth to Mewtwo...)
Actually, it depends on whether you go by the anime or the games. In the anime Mewtwo was cloned from a fossil of an ancient Mew found in South America... supposedly...
In the games the scientists at Pokemon Mansion captured a live Mew and got it pregnant, then most likely tampered with the embryo.
Either way Mewtwo goes insane and blows up most of the science facility before escaping.