Mendel identified the trait of dominance in certain genes. He observed that when crossing pea plants with contrasting traits (e.g., purple flowers and white flowers), one trait would always appear in the first generation (F1) while the other trait seemed to disappear. He called the trait that appeared dominant and the trait that seemed to disappear recessive.

This led him to propose the concept of alleles, alternative forms of a gene, and that each individual inherits two alleles for each trait, one from each parent. He also realized that the recessive allele is still present in the F1 generation, even though it doesn't manifest. It can reappear in the F2 generation when two F1 individuals are crossed.

This discovery of dominance was a crucial breakthrough in understanding how traits are passed from parents to offspring and paved the way for modern genetics.