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STD 12 - 4. principal of inheritance and variation — Biology STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceBiologySTD 12 - 4. principal of inheritance and variation1 Mark
MCQ
Select the incorrect statement with regard to haemophilia.
  • ✓
    It is a dominant disease.
  • B
    A single protein involved in the clotting of blood is affected.
  • C
    It is a sex­linked disease.
  • D
    It is a recessive disease.
✓

Answer

Correct option: A.
It is a dominant disease.
a
(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 $(X^hY)$.

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