(a): Haemophilia is sex­linked disease which is also known as bleeder’s disease as the patient will continue to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting due to absence of antihaemophiliac globulin or factor $VIII$ (haemophilia -$A$) and plasma thromboplastin factor $IX$ (haemophilia-$B$, Christmas disease) essential for it. As a result of continuous bleeding the patient may die of blood loss. It is geneticallydue to the presence ofa recessive sex linked gene $h$, carried by $X$-­chromosome. A female becomes haemophiliac only when both of her $X$-­chromosomes carry the gene $(X^hX^h)$. However, such females generally die before birth because the combination of these two recessive alleles is lethal. A female having only one allele for haemophilia $(XX^h)$ appears normal because the allele for normal blood clotting present on the other X­chromosome is dominant. Such females are known as carriers. In case of males, a single gene for the defect is able to express itself as the $Y$­ chromosome is devoid of any corresponding allele $(X^hY)$.