The garden pea has closed flowers and can self-fertilize, but also allow for manual cross-pollination.
Mendel studied inheritance of a single character (or trait, such as flower color) by cross-pollinating two true-breeding varieties of the pea.
This is called a monohybrid cross.
When pollen from a white flower fertilizes eggs of a purple flower,
the first-generation hybrids (F1) all have purple flowers.