Self-pollination

The pollination of a flower by its own pollen. Peas pollinate themselves before the flower opens, so stray pollen rarely gets in and a cross made by hand stays the cross intended.

This is what made peas usable for Mendel's experiments: since stray pollen rarely gets in, a cross made by hand is kept as made. He obtained 34 varieties, grew them for two years to make sure each bred true, and used 22 of them^[1].

When the hybrids were left to pollinate themselves, the traits split in the next generation. For seed shape, 253 plants gave 7,324 seeds, 5,474 round and 1,850 wrinkled, or 2.96 to 1^[1].