Final answer:
Mendel was able to assume his plants were truebred because he selectively bred only those pea plants that produced offspring that were the same as the parents by self-crossing them. This process allowed Mendel to avoid the appearance of unexpected traits in the offspring. He also took advantage of the short life cycle and ability for large-scale cultivation of garden peas to study several generations and draw meaningful conclusions.
Step-by-step explanation:
Mendel was able to assume his plants were truebred because he selectively bred only those pea plants that produced offspring that were the same as the parents, a process known as self-crossing or self-mating. This was possible because garden peas naturally self-fertilize, meaning that pollen encounters ova within individual flowers. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true-breeding.
Additionally, garden peas grow to maturity within one season, allowing Mendel to evaluate several generations over a relatively short time. By cultivating large quantities of garden peas simultaneously, Mendel was able to conclude that his results did not come about simply by chance.