Haemophilia is a sex-linked recessive disease controlled by a gene present on the X-chromosome. Males (XY) have only one X chromosome; therefore, a single copy of the mutant allele is sufficient for the disease to be expressed. Females (XX) must be homozygous recessive (have the mutant allele on both X chromosomes) to show the disease. The possibility of a female becoming haemophilic is extremely rare because the mother of such a female has to be at least a carrier and the father should be haemophilic .