Mendel had no "true" monohybrids.

Mendel's experiments in hybridizing peas started in 1856 and ended in 1863. When one attempts to fit the described experiments--in the sequence indicated in the paper--into those limits some serious implications emerge with regard to parental types needed and the scheduling of the experiments. If Mendel had to develop the parental types of his monohybrid and dihybrid crosses and then do the experiments he described, 8 years would not have been a sufficient amount of time. We believe that his original true-breeding varieties differed in more than one trait on the basis of a logical analysis of breeding relationships. It follows first that his "monohybrid" experiments were performed with varieties in several traits but that in each offering he concentrated his attention on only one, second that he had the parental lines for his dihybrid and trihybrid experiments on hand before he started those experiments.