U may get into trouble 6-7generations down the line with inbreeding, but essentially it is line breeding that pure strains are after. It's like you would breed a dalmation dog with another dalmation dog to get a dalmation puppies. U would not cross a dalmation with a duschund for instance and if you keep crossing eventually u'll end up with mutts on the street....

A lot of work has been put to create pure strains of guppies... just like pure breed of dogs.