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One of the crucial factors in the evolution of herbicide resistance in weeds is the mode of inheritance. Experiments were conducted to determine the inheritance of glyphosate resistance in a population of perennial ryegrass (Lolium perenne) caused by restricted glyphosate translocation. First, the degree of dominance of the glyphosate resistance trait was evaluated by generating first filial generation families through pair-crossing resistant with susceptible plants. Dose-response experiments showed that these first generation families had an intermediate level of glyphosate resistance compared with that of the parent plants. The phenotypic segregation of the resistance trait was then investigated by backcrossing the first generation offspring with individuals from the original susceptible population. Results obtained from spraying the resulting backcrossed families with two rates of glyphosate gave segregation values that best fitted the one-gene model. Hence, glyphosate resistance in perennial ryegrass caused by the restricted herbicide translocation trait is controlled by a single nuclear gene with incomplete dominance. [ABSTRACT FROM AUTHOR]