one environmental extreme than the other, rather than being approximately equivalent at both ends of the distribution (but with genotypic effects in opposite directions) as is the case in the crossover effect illustrated in Figure 1b. In general, it is assumed that main effects of genotype will not be detected in crossover interactions, but this will actually depend on the frequency of the different levels of the environment. This is also true of fan-shaped interactions, but to a lesser degree. Note that the crossover depicted in Figure 1b does indicate a main effect of the environment. However, here too substantial variation can be imagined. For example, if the crossover took the shape seen in Figure 1c, assuming approximately equal distributions of environmental levels, you would not find a main effect for genotype or environment. Interactions of this sort are assumed to be relatively rare.
