(a) Haemophilia is due to the presence of a recessive defective allele on X – chromosome.
(b) A female with XX – chromosome must be homozygous for the disease to appear.
(c) She must receive one of the alleles from her haemophilie father (XhY) and the other from her mother who is also an haemophilie or at least a carrier i.e. the heterozygous for the disease XXh.