Genetic carrier

A hereditary carrier (or just carrier), is a person or other organism that has inherited a recessive allele for a genetic trait or mutation but does not display that trait or show symptoms of the disease. Carriers are, however, able to pass the allele onto their offspring, who may then express the genetic if they inherit the recessive allele from both parents. The chance of two carriers having a child with the disease is 25%. This phenomenon is a direct result of the recessive nature of many genes.

Queen Victoria, and her daughters Princesses Alice and Beatrix, were carriers of the X-linked hemophilia gene (an abnormal allele of a gene, necessary to produce one of the blood clotting factors). Both had children who continued to pass on the gene to succeeding generations of the royal houses of Spain and Russia, into which they married. Since males only have one X chromosome, males who carried the altered gene had hemophilia. Females have two X chromosomes, so one copy of an X-linked recessive gene would cause them to be an asymptomatic carrier. These females simply passed it to half of their children.

Up to 1 in 25 individuals of Northern European ancestry may be considered carriers of mutations (CFTR genes) that can lead to cystic fibrosis. The disease appears only when two of these carriers have children, as each pregnancy between them will have a 25% chance of producing a child with the disease. However, it is also thought that carriers of CF may be more resistant to diarrhea during typhoid fever or cholera, and are therefore not truly asymptomatic. This resistance leads to increased fitness of the carriers, known as a heterozygote advantage, and thereby increases the frequency of the altered genes in the population. Although only about 1 of every 3,000 Caucasian newborns has CF, there are more than 900 known mutations of the gene that causes CF. Current tests look for the most common mutations.

...
Wikipedia