In genetics, the term carrier describes an organism that carries two different forms of a recessive gene (alleles of a gene linked to a recessive trait) and is thus heterozygous for that the recessive gene. Although carriers may act to convey and maintain recessive genes within a population by passing them on to offspring, the carriers themselves are not affected by the recessive trait associated with the recessive gene.

Although a carrier's genome contains a particular mutant allele, another gene (e.g., a dominant gene), or series of genetic mechanisms act to prevent the observable expression of that mutant allele (phenotypic expression). If, for example, at the genetic level an organism had a genotype (T, t), with the capital letter "T" designating a completely dominant allele and the lowercase letter "t" representing the recessive allele, that organism would be express the observed trait associated with "T" and be...