In genetics, the term carrier describes an organism that carries two different forms (alleles) 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 express the observed trait associated with "T" and be a carrier for the recessive gene designated by "t." In contrast, the human blood type AB presents an example of allele codominance because the allele IA and IA allele are both expressed and contribute to the phenotype (blood group AB).
Because heterozygous organisms carry contain different forms (alleles) of a particular gene, diploid carriers produce sex cells (gametes) by the process of cell meiosis. Accordingly, heterozygous organisms produce gametes that contain different copies of the genes for which they are heterozygous. With regard to a (T, t) genotype, such an diploid organism would produce equal numbers of gametes that carried a single "T" allele or a single "t" allele.
At the observable level, an individual may, for example, act to convey the sickle cell gene but remain unaffected by sickle cell disease that strikes those who are homozygous for the sickle cell gene (i.e., carry two copies of the recessive sickle cell allele).
Under some conditions, a carrier may actually be more fit for a particular environment. Carriers who benefit from this heterozygote superiority or advantage are able to pass on and maintain a particular recessive allele within a population. In the case of sickle cell, the heterzygote carrier has a greater resistance to some forms of malaria. Accordingly, in malaria stricken areas, carriers of sickle cell disease avoid (in greater numbers) the selective disadvantages of malaria.
Studies of patients of Ashkenazi Jewish heritage (Jews of Eastern European descent), indicate that as many as one in seven individuals acts as a carrier of at least one of several different genetic diseases. Although some of these diseases are potentially fatal, the carriers of these diseases remain observably healthy individuals and show no signs of being affected with the disease related to the particular gene they carry.
Geneticists and physicians have developed a number of screening tests (carrier screening) to identify individuals who may be carriers for a particular gene.