Zoology - Principles of Inheritance and Variation Question with Solution | TestHub

Haemophilia being a sex chromosome related recessive disorder has its gene present on $\mathrm{X}$ chromosome. Since males have one $\mathrm{X}$ and one $\mathrm{Y}$ chromosome, and the genes present on $\mathrm{X}$ don't have counter genes on $\mathrm{Y}$, only one affected gene (to be present on $\mathrm{X}$ ) is required for the disease in males. While in females, there are $2\mathrm{X}$ chromosomes. For the disease to occur both the $\mathrm{X}$ chromosomes should have the affected genes which is a rare chance. For this to be possible, the father of the female must be diseased while the mother must atleast be a carrier.