Write a short note on Haemophilia.

(N/A) Haemophilia is a sex-linked recessive disorder that is transmitted from an unaffected carrier female to some of the male progeny.
$1$. Genetic Basis: The gene for haemophilia is located on the $X$ chromosome. Since it is recessive,females $(XX)$ require two copies of the defective gene to express the disease,whereas males $(XY)$ only require one copy.
$2$. Physiological Effect: The disorder results in a defect in the blood clotting mechanism. $A$ single protein that is part of the cascade of proteins involved in the clotting of blood is affected.
$3$. Clinical Manifestation: Due to the defect,a simple cut results in non-stop bleeding in an affected individual.
$4$. Inheritance Pattern: The possibility of a female becoming a haemophilic is extremely rare because the mother of such a female has to be at least a carrier and the father should be haemophilic (unviable in the later stage of life).