A double recessive gene contains recessive alleles inherited from both parents. Alleles are variations of a gene controlling a particular characteristic such as eye color. The presence of both recessive alleles results in the expression of the recessive trait.
An organism born of sexual reproduction has a genetic makeup, or genotype, that is a mixture of the genotypes of both its parents. Two such organisms therefore produce offspring containing genetic information from four sources. Each parent passes on one allele, so the child of a male parent with alleles Aa and a female parent with alleles Bb could inherit gene AB, Ab, aB, or ab. In this example, if lowercase "a" and "b" represent recessive alleles, the combination "ab" forms a double recessive gene.
An individual's genotype contains its complete genetic code, while the genetic qualities manifested in the individual are called its phenotype. The terms "dominant" and "recessive" refer to the likelihood of an allele being expressed in the phenotype. A dominant allele finds expression if inherited from only one parent, whereas a recessive allele must be inherited from both parents to manifest. Note, however, that these terms represent an oversimplification; many genetic traits result from alleles that share dominance in a heterozygous individual (one carrying both dominant and recessive alleles).