How did Gregor Mendel’s results show that genes occur in pairs?

Gregor Mendel deduced that genes occur in pairs through his quantitative experiments with pea plants. By crossing pure-breeding varieties, he observed that the first generation (F1) hybrids expressed only the dominant trait, while the recessive trait appeared to vanish.12

However, when he allowed these F1 hybrids to self-pollinate, the recessive trait reappeared in the second generation (F2) in a consistent 3:1 ratio. Mendel concluded that the recessive trait must have been present but masked in the F1 generation. This led him to hypothesize that each plant carries two discrete hereditary units—now known as genes—for each trait.12

He further deduced that these pairs separate during the formation of gametes, ensuring that each sex cell receives only one member of the pair. This concept of paired units and their subsequent separation is the foundation of his law of segregation, which explains how traits are inherited and why they reappear in predictable proportions.123