Haemophilia (also known as bleeder disease) is an popular example of sex linked inheritance in human beings. Haemophilia is either of two hereditary disorders in which the blood clots very slowly, due to a deficiency of either of two coagulation factors haemophilia A, due to deficiency of factor VIII (antihaemophilic factor); or haemophilia B, due to deficiency of factor IX (Christmas factor). The patient may experience prolonged bleeding following any injury or wound, and in severe cases there is spontaneous bleeding into muscles and joints. Haemophilia is controlled by a sex-linked gene, which means that it is almost exclusively restricted to males; women can carry the disease - and pass it on to their sons - without being affected themselves. The genes encoding factors VIII and IX have been used in gene therapy trials for haemophilia.
Mutation of a structural gene on chromosome 15 is the molecular basis of Marfan syndrome. This disease is due to domiant mutation resulting in the production of abnormal form of connective tissues and characteristic extreme looseness of joints.