Pea plants were excellent model organisms because they can either cross-pollinate or self-pollinate, all inheritance patterns were simple, easy to grow, and most of the traits studied had no intermediate stage.
Peas plants are both allogamous (cross-pollinate) and autogamous (self-pollinate), although the likelihood of cross-pollinating is low.
The phenotypic traits used by Mendel had simple inheritance patterns controlled by one single gene or a few genes.
Mendel used pea plants because they have easily identifiable traits such as the form of ripe seed (smooth or wrinkled), the color of seed albumen (yellow or green), etc.
The phenotypic traits studied by Mendel had no intermediate stage, i.e., they are qualitative traits.
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