Multipartite plant viruses were discovered because of discrepancies between the observed dose response and predictions of the independent action hypothesis (IAH) model. Theory suggests that the number of genome segments predicts the shape of the dose response, but a rigorous test of this hypothesis has not been reported. Here Alfalfa mosaic virus (AMV), a tripartite Alfamovirus, and transgenic Nicotiana tabacum plants expressing none (wild-type), one (P2) or two (P12) viral genome segments were used to test whether the number of genome segments necessary for infection predicts dose response. Dose response for wild-type plants was steep and congruent with predicted kinetics for a multipartite virus, confirming previous results. Moreover, for P12 plants the data support the IAH model, showing that the expression of virus genome segments by the host plant can modulate the infection kinetics of a tripartite virus to those of a monopartite virus. However, the different types of virus particles occurred at different frequencies, with a ratio 116:45:1 (RNA1:RNA2:RNA3), which will affect infection kinetics and required analysis with a more comprehensive infection model. This analysis showed that each type of virus particle has a different probability of invading the host plant, both at the primary- and systemic-infection levels. Whilst the number of genome segments affects dose response, taking into consideration differences in the infection kinetics of the three types of AMV virus particles results in a better understanding of the infection process.