page_title: Genetic carrier
section_title: Carriers in gonosomal inheritances
hierarchical_section_title: Genetic carrier / Carriers in gonosomal inheritances
caption_attribution_description: Deutsch: Gonosomal-rezessiver Erbgang der Bluterkrankheit bei den Nachkommen der Königin Victoria zur Stammbaumanalyse
context_page_description: A hereditary 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.
context_section_description: Gonosomeal 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. If a man has a certain recessive genetic disposition on his X chromosome, this is called hemizygot and it gets phenotypically expressed. Also a recessive genetic disposition on his Y chromosome - also hemizygot - can come to expression, because there is no homologous chromosome with an allele, which could overlay it. If there is no genetic information on the Y chromosome for a certain trait, the effect of the Y is neutral and the allele on his X chromosome, which would be recessive in a heterozygous woman, can now alone come to expression.
  Women have two homologous sex chromosomes (XX). Therefore, women can be carriers of X-linked genes. Examples of traits inherited via the X chromosome are color blindness and the most common hereditary form of Haemophilia. Men are affected much more often than women.
  Queen Victoria, and her daughters Princesses Alice and Beatrix, were carriers of the  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 B. 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.
  Gonosomal dominant inheritances are also known. There are no carriers since owners of a dominant hereditary disposition phenotypically express the trait in each case.
caption: Inheritance by female carriers
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