In both American cotton (Gossypium hirsutum) and Egyptian cotton (G. barbadense) petal spot is a dominant character over no petal spot. When the cross is made within the species, the F2 progeny segregates in a 3:1 ratio with petal spot character in 75% of the population. However, no clear cut ratio is obtained in respect of the same flower character, when the cross is made reciprocally between the above two species. What is the explanation of this fact?