Haemophilia is a famous blood disease. Its fame comes from the children of Queen Victoria and their offspring.
The symptoms are that blood fails to clot. The smallest wound or tooth extraction can prove fatal. A bump will not lead to a bruise but large, internal bleeding.
Nowadays sufferers are treated with a protein extracted from the blood of donors. Regular injections of clotting factor 8 can allow the patient's blood to clot normally.
The genetic basis of the disease is that the damaged allele occurs on the X sex chromosome.
Therefore it is sex-linked.