What did Gregor Mandel discover about recessive traits?

Gregor Mendel discovered that recessive traits do not manifest in an organism's phenotype unless two copies of the recessive allele are present. In his pea plant experiments, he observed that when a dominant allele is paired with a recessive allele, the dominant trait dominates the phenotype. Only when both alleles are recessive does the recessive trait become visible in the offspring. This foundational principle of inheritance laid the groundwork for modern genetics.