Genetic carrier

A genetic carrier (or just carrier), is a person or other organism that has inherited a genetic trait or mutation, but who does not display that trait or show symptoms of the disease. They are, however, able to pass the gene onto their offspring, who may then express the gene. This phenomenon is a direct result of the recessive nature of many genes.

For example, the daughters of Queen Victoria, the princesses Alice and Beatrix, were carriers of the X-linked hemophilia gene (more precisely, an abnormal allele of a gene necessary to produce one of the blood clotting factors). Both had children who continued to pass the gene to succeeding generations of the royal houses of Spain and Russia, into which they married. Males who carried the altered gene had hemophilia, while females simply passed it to about half of their children.

Up to 1 in 25 individuals of Northern European ancestry may be considered carriers of mutations that could lead to Cystic Fibrosis. The disease appears only when two of these carriers have children (25% of whom will be affected by the disease). However, it is also thought that carriers may be more resistant to diarrhea during typhoid fever or cholera, and are therefore not truly asymptomatic. This resistance leads to increased survival of the carriers, thereby increasing the frequency of the altered genes in the population.