Genetic redundancy

Genetic redundancy is when two or more codons in a gene code for the same protein, so the inactivation of one of these codons due to mutation has little or no effect to the organism.

In classical Mendelian genetics experiments this manifests as a form of epistasis where the vast majority of the progeny of a dihybrid cross have normal or near normal phenotypes and only the double homozygous recessive genotype shows a severe mutant phenotype.

Genetic redundancy is not associated with gene duplication and redundant genes do not mutate faster than essential genes. However, multiple "individually" harmful mutations can accumulate on a redundant gene, leading eventually to a beneficial gene that would not otherwise be accessible. One example of this is the inferred evolution of hemoglobin from myoglobin in vertebrate animals.