Ahereditary carrier (genetic carrier or justcarrier), is a person or other organism that hasinherited a recessive allele for a genetic trait or mutation but usually does not display thattrait or show symptoms of thedisease. Carriers are, however, able topass theallele onto their offspring, who may then express the genetic trait.
Autosomal dominant-recessiveinheritance is made possible by the fact that the individuals of most species (including all higher animals and plants) have twoalleles of most hereditary predispositions because thechromosomes in thecell nucleus are usually present in pairs (diploid). Carriers can be female or male as the autosomes are homologous independently from the sex.
In carriers the expression of a certain characteristic is recessive. The individual has both a genetic predisposition for thedominant trait and a genetic predisposition for the recessive trait, and the dominant expression prevails in thephenotype. In an individual which isheterozygous regarding a certain allele, it is not externally recognisable that it also has the recessive allele. But if the carrier has a child, the recessive trait appears in the phenotype, in case the descendant receives the recessiveallele from both parents and therefore does not possess the dominant allele that would cover the recessive trait. According toMendelian Law of Segregation of genes an average of 25% of the offspring becomehomozygous and express the recessive trait. Carriers can either pass on normal autosomal recessive hereditary traits or an autosomal recessivehereditary disease.
Gonosomal recessive genes are also passed on by carriers. The term is used inhuman genetics in cases of hereditary traits in which the observed trait lies on the femalesex chromosome, theX chromosome. These aresex-linked genes. The carriers are alwayswomen. Women have two homologous sex chromosomes (XX).Men cannot be carriers because they only have one X chromosome. If a man has a certain recessive genetic disposition on his X chromosome, this is calledhemizygous and it gets phenotypically expressed.
Although the Y chromosome is not a really homologous chromosome and carries relatively little genetic information compared to X chromosomes, a genetic component on the Y chromosome can come to expression because there is no homologous chromosome with an allele which could overlay it.
Examples of traits inherited via the X chromosome arecolor blindness and the most common hereditary form ofhaemophilia which therefore affect men much more often than women.[1][2]
Queen Victoria, and her daughters Princesses Alice and Beatrix, were carriers of thehemophilia 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 ofSpain andRussia, into which they married.[3] Since males only have one X chromosome, males who carried the altered gene had hemophilia B. Those female children who inherited the altered gene were asymptomatic carriers who also would have passed it to half of their children.
Gonosomal dominant inheritances are also known. There areno carriers since owners of a dominant hereditary disposition phenotypically express the trait in each case.