In Haemophilia or Bleeders' disease, females are alwas carrier can only males contract the disease. Why?

Explanation

Haemophilia is is sex-linked trait caused by a recessive gene (say h) located in the X chromosome. Man has one X chromosome and one Y chromosome. The gene for haemophilia, being on X chromosome, is able to produce the defect in make because there is no homologue of this gene in the Y cyhromosomes to check its expression.
Woman, on the other hand, has two X chromosomes. For her to be haemophiliac, it is necesary that each X chromosome has a gene for haemophilia. However, such females are usually not born as the combination of two recessive genes in lethal (fatal). If only one X chromosome bears a gene for haemophilia, its dominant homolgous gene ihn the other X chromosome will check the expression of the recessive gene. Such females are called carriers.