Why did Gregor Mendel conclude that hereditary traits do not blend together?

Gregor Mendel concluded that hereditary traits do not blend together based on his quantitative experiments with pea plants. While his predecessors often observed that hybrids appeared intermediate between their parents—supporting a "blending" theory of inheritance—Mendel’s rigorous methodology revealed a different reality.12

His conclusion was driven by two key observations:

• Reappearance of Recessive Traits: When Mendel crossed pure-breeding varieties (e.g., yellow-seeded and green-seeded plants), the first-generation (F₁) hybrids displayed only the dominant trait. However, when he allowed these F₁ plants to self-pollinate, the recessive trait reappeared in the second generation (F₂) as pure and uncontaminated as it was in the original parents.13
• Mathematical Ratios: By making exact counts of the plants, Mendel discovered that these traits appeared in consistent, predictable proportions, such as the 3:1 ratio. This quantitative data demonstrated that the underlying "factors" (now known as genes) remained discrete units that segregated during the formation of gametes rather than mixing like fluids.13

These findings allowed Mendel to disprove the "blood theory" of inheritance, establishing that traits are transmitted as distinct, non-blending units.12