Class 12 NEET Biology Principles of Inheritance and Variation Questions Part 2

Haemophilia is sex-linked 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 genetically due to the presence of recessive sex linked gene $h,$ carried by ${X}$ -chromosome. A female becomes haemophiliac only when both of her X-chromosomes carry the gene $( {X}^{ {h}} {X}^{ {h}}) .$ 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 ${X}$ -chromosome 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 Ychromosome is devoid of any corresponding allele $( {X}^{ {h}} {Y})$