Haemophilia is sex-linked disease which is also known as bleeder’s disease as the patient will continue to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting due to absence of antihaemophiliac globulin or factor VIII (haemophilia – A) and plasma thromboplastin factor IX (haemophilia–B, Christmas disease) essential for it. As a result of continuous bleeding the patient may die of blood loss. It is genetically due to the presence of a recessive sex linked gene h, carried by X-chromosome. A female becomes haemophiliac only when both of her X-chromosomes carry the gene (X^hX^h).

However, such females generally die before birth because the combination of these two recessive alleles is lethal. A female having only one allele for haemophilia (XX^h) appears normal because the allele for normal blood clotting present on the other X-chromosome is dominant. Such females are known as carriers. In case of males, a single gene for the defect is able to express itself as the Y-chromosome is devoid of any corresponding allele (X^hY).