When a tall pea plant with round seeds (TTRR) was crossed with a dwarf pea plant with wrinkled seeds (ttrr), where T stands for dominant tall and t for recessive dwarf height, and R for dominant round and r for recessive wrinkled seed shape, the results were as follows: The individual F1 populations all possessed tall, round seeds. However, self-pollination among F1 individuals resulted in a phenotypic ratio of  9:3:3:1. This ratio is caused by which of the following principles?