A female will be affected with haemophilia only in the rare circumstance that she inherits mutated X chromosomes from both a haemophiliac father and a carrier or a haemophiliac mother. The daughter of a male haemophiliac will always inherit his mutation, while a son cannot ever inherit it. Each child of a carrier will have a 50% chance of inheriting their mother's mutation, of being a haemophiliac (sons) or carrier (daughters). If their sole X chromosome contains the haemophilia mutation they possess no second copy to provide for normal function, as in carrier females. Males possess only a single X chromosome, inherited from their mother, having received a Y chromosome from their father instead of a second X. Such a female, with normal clotting but possessing a single mutated copy of the gene, is called a carrier. A female who inherits a mutated copy on one X chromosome has also inherited a second X chromosome from the other parent that is likely to carry a non-mutated copy of the gene, capable of directing appropriate clotting. Females have two X chromosomes, and hence redundant copies of the blood clotting factor gene located on them. This is because the trait is recessive, meaning that only one correctly functioning copy of the blood clotting factor gene is necessary for normal clotting. The sex-linked X chromosome bleeding disorder manifests almost exclusively in males, even though the genetic mutation causing the disorder is located on the X chromosome and can be inherited from the mother by male children or from either mother or father by female children. The presence of haemophilia B within the European royal families was well-known, with the condition once popularly known as 'the royal disease'. Tests on the remains of the Romanov imperial family show that the specific form of haemophilia passed down by Queen Victoria was probably the relatively rare haemophilia B. Victoria's youngest son, Prince Leopold, Duke of Albany, also had the disease, though none of her three elder sons did.

Queen Victoria and her husband, Prince Albert, of the United Kingdom, through two of their five daughters – Princess Alice and Princess Beatrice – passed the mutation to various royal houses across the continent, including the royal families of Spain, Germany, and Russia. Haemophilia figured prominently in the history of European royalty in the 19th and 20th centuries. Preponderance of a genetic disorder throughout European royalty Queen Victoria's descendants with haemophilia and known female carriers Inheritance by female carriers

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