Final answer:
The key advantage of using purebred pea plants in Mendel's genetic experiments was the elimination of unexpected traits, enabling him to observe clear inheritance patterns due to their true-breeding nature and the ability to grow many generations within a short time frame.
Step-by-step explanation:
The most important advantage of using purebred pea plant varieties in Mendel's work was that these true-breeding plants always produce offspring that resemble the parent, thereby eliminating unexpected traits and enabling Mendel to conduct controlled genetic experiments. Pea plants were ideal for Mendel's experiments because they self-fertilize, which means their flower petals remain sealed tightly during pollination, preventing crossbreeding and maintaining pure lines. Additionally, pea plants mature within one season, allowing for the observation of many generations in a short amount of time, and can be grown in large quantities to ensure statistical significance. This made it possible for Mendel to observe clear patterns of inheritance, leading to the formulation of the foundational laws of genetics.