Genetic carrier
A genetic carrier is a person or other organism that has inherited
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 a genetic trait
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 or mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

, but who does not display that trait or show symptoms of the disease. They are, however, able to pass
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...

 the gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 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
Victoria of the United Kingdom
Victoria was the monarch of the United Kingdom of Great Britain and Ireland from 20 June 1837 until her death. From 1 May 1876, she used the additional title of Empress of India....

, 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
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

 and Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

, 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
Cystic fibrosis
Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...

. 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 may be more resistant to diarrhea during typhoid fever
Typhoid fever
Typhoid fever, also known as Typhoid, is a common worldwide bacterial disease, transmitted by the ingestion of food or water contaminated with the feces of an infected person, which contain the bacterium Salmonella enterica, serovar Typhi...

 or cholera
Cholera is an infection of the small intestine that is caused by the bacterium Vibrio cholerae. The main symptoms are profuse watery diarrhea and vomiting. Transmission occurs primarily by drinking or eating water or food that has been contaminated by the diarrhea of an infected person or the feces...

, and are therefore not truly asymptomatic. This resistance leads to increased survival
Heterozygote advantage
A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...

 of the carriers, thereby increasing the frequency of the altered genes
Cystic fibrosis transmembrane conductance regulator
Cystic fibrosis transmembrane conductance regulator is a protein that in humans is encoded by the CFTR gene.CFTR is a ABC transporter-class ion channel that transports chloride and thiocyanate ions across epithelial cell membranes...

in the population.

Genetic carrier testing can be used to tell if a person carries one or more mutations of the CF gene and how many copies of each mutation. The test looks at a person’s DNA (genetic material), which is taken from cells in a blood sample or from cells that are gently scraped from inside the mouth.

Although only about one of every 3,000 Caucasian newborns has CF, there are more than 1,000 known mutations of the gene that causes CF. Current tests look for the most common mutations.

The mutations screened by the test vary according to a person's race or ethnic group, or by the occurrence of CF already in the family. More than 10 million Americans, including one in 29 Caucasian Americans, are carriers of one mutation of the CF gene. In other races or ethnicities, one in 46 Hispanic Americans, one in 65 African Americans and one in 90 Asian Americans carry a mutation of the CF gene.

If you have a relative with CF or who is known to carry a mutation of the CF gene, your chances of carrying a mutation are greater because of your family's history. If you are pregnant or planning to have a child, you should discuss this test and the results with a health professional who is knowledgeable about genetic testing, such as a genetic counselor.
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