Gene on X-chromosome. Hemophilia a is a hereditary bleeding disorder where there is a deficiency in clotting factor viii (factor viii is needed for blood clotting). It is inherited on the x-chromosome (sex-linked). Men only have 1 "x", so if their "x" is mutated, they will have the disease. Women have 2 "x's", so they can have 1 mutated "x" and 1 normal "x", and be a carrier of the disease, with mild or no symptoms.
Many. In one study of 200 affected genes, seven different mutations were demonstrated. Four of them were transpositions of single bases, of which three transformed a codon for arginine into a stop codon, the fourth mutation resulted in substitution of a single amino acid and mild hemophilia. The other 3 mutations involved deletion of several thousand nucleotides and produced severe hemophilia.