A carrier is an individual who carries and is capable of passing on a genetic mutation associated with a disease and may or may not display disease symptoms. Carriers are associated with diseases inherited as recessive traits. In order to have the disease, an individual must have inherited mutated alleles from both parents. An individual having one normal allele and one mutated allele does not have the disease. Two carriers may produce children with the disease.
In order to understand the meaning of the word "carrier", we have to remember that each individual carries two copies of any gene. A carrier is an individual that has a change in one of those two copies. When two individuals that are carriers meet and produce an offspring, if that offspring inherits the changed or misspelled copy of the gene from each parent, that individual, that offspring, will have a recessive disorder and will have clinical features of a recessive disorder. In the past it was felt that carriers or individuals that carried one copy, or one misspelled copy of a gene, showed no clinical features. However, more recently there's increasing evidence that in some recessive conditions, the carriers actually have some clinical manifestations, and this story continues to unfold.- Donna Krasnewich, M.D., Ph.D.