In Neurospora crassa, a mutation in the poky gene results in slow growth (poky). The results of four crosses are given:
(1) wild-type ♀ × wild-type ♂ → All progeny wild-type
(2) wild-type ♀ × poky ♂ → All progeny wild-type
(3) poky ♀ × wild-type ♂ → All progeny poky
(4) poky ♀ × poky ♂ → All progeny poky
Which one of the following explains the inheritance mode of poky?