Hereditary carrier

A hereditary carrier (genetic carrier or just carrier), is a person or other organism that has inherited a recessive allele for a genetic trait or mutation but usually 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 trait. Autosomal dominant-recessive inheritance is made possible by the fact that the individuals of most species (including all higher animals and plants) have two alleles of most hereditary predispositions because the chromosomes in the cell 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 the dominant trait and a genetic predisposition for the recessive trait, and the dominant expression prevails in the phenotype. In an individual which is heterozygous 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 recessive allele from both parents and therefore does not possess the dominant allele that would cover the recessive trait. According to Mendelian Law of Segregation of genes an average of 25% of the offspring become homozygous and express the recessive trait. Carriers can either pass on normal autosomal recessive hereditary traits or an autosomal recessive hereditary disease. Gonosomal recessive genes are also passed on by carriers. The term is used in human genetics in cases of hereditary traits in which the observed trait lies on the female sex chromosome, the X chromosome. The carriers are always women. Men cannot be carriers because they only have one X chromosome. The Y chromosome is not a really homologous chromosome. For this reason, the genetic make-up of the observed trait is not twofold.