Mendel conducted experiments crossing pea plants to study heredity. He found that pea plants have traits determined by discrete units (now known as genes and alleles) that are passed from parents to offspring. When crossing pea plants with different expressions of a trait (e.g. tall vs. short stems), the offspring of the first filial generation (F1) always expressed one trait, while a quarter of the offspring of the second filial generation (F2) expressed the recessive trait. This supported his theory that traits are passed to offspring through discrete inherited factors.