Mendel called a trait that disappears in the first generation recessive.
In his pea experiments, Mendel crossed true-breeding plants with contrasting traits. All of the F1 offspring displayed one parental trait, while the other seemed to vanish. When the F1 hybrids reproduced, the hidden trait reappeared in the F2 generation, usually in about one-quarter of the offspring.
For example, when yellow-seeded peas were crossed with green-seeded peas, the F1 generation showed yellow seeds. Green seeds reappeared in the next generation, demonstrating that the underlying hereditary factor had not been destroyed; it had simply been masked. Mendel described the visible trait as dominant and the masked trait as recessive.
Mendel used the language of “factors,” not the modern terms gene and allele, which were introduced later. Today, a recessive allele is generally expressed in the phenotype only when no dominant allele masks it, often when two recessive copies are inherited.