In diploid organisms, the attribute of a specified character (or phenotype) is said to be recessive if it is masked when in the presence of a dominant allele. A recessive phenotype is expressed only in homozygotes (or, for X-linked traits, also in hemizygotes). Strictly speaking, it is the phenotype that is recessive (or dominant), not the allele; however, the term "recessive gene" is widely, if loosely used.

Gregor Mendel recognized the dominant and recessive aspects of the traits he studied in garden peas in the mid nineteenth century. Those traits that become latent in the process of the hybridization he described as recessive, whereas those that constituted the character of the hybrid were seen as dominant. Mendel studied seven characters of peas, each with two recognizable traits; for example, round or wrinkled seeds, dwarf or tall stems, and several others. Starting with...