Carrier

In genetic situations where two copies (alleles) of a gene are present (i.e., autosomal genes and X-linked genes in females), a 'carrier' has an abnormal (generally mutationally defective) allele at one of the two alleles. This mutant allele usually does not cause an abnormal phenotype and is said to be recessive. However, a carrier may pass the chromosome containing the mutant allele to offspring and when this is combined with a similarly mutant allele from the other parent (for autosomal genes) or is present in an uncomplemented state (an X-linked mutation in a male), the defect caused by the gene will be manifest. A wide variety of gene variations is becoming recognized through genomic sequencing and the carrier state can be detected by several methods.