An enterprising pet store owner is attempting to create a pure strain of ultra-dwarf mice to sell as novelty pets.

An enterprising pet store owner is attempting to create a pure strain of ultra-dwarf mice to sell as novelty pets. he breeds 10 pairs of ultra-dwarf mice and counts the different phenotypes seen in the offspring. (A wild-type mouse litter is 10 mice.) In the F1 generation, 50 are ultra-dwarf, and 25 are wild-type. Each time he breeds two ultra-dwarves he gets the same ratio.–What is going on?–Will he ever get a pure strain? If not, why?