The reason why haemophilia is more commonly observed in human males than in females is due to

The correct answer is the disease is due to X-linked recessive mutation.

To understand why haemophilia is more commonly observed in human males than in females, we need to look at how the disease is inherited genetically:
1. Sex Chromosomes in Humans:
Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
2. Nature of the Mutation:
Haemophilia is a genetic disorder caused by a recessive mutation on the X chromosome (an X-linked recessive trait). Let us denote the normal allele as X^H and the mutant (haemophilic) allele as X^h.
3. Expression in Males (XY):
Since males possess only one X chromosome, they inherit only a single allele for X-linked genes. If a male inherits the mutant allele (X^hY), he does not have a second X chromosome with a normal allele to mask it. Therefore, a single copy of the mutant allele is sufficient to cause the disease in males.
4. Expression in Females (XX):
For a female to exhibit haemophilia, she must inherit two copies of the mutant allele (X^hX^h) — one from her mother and one from her father. If she inherits only one mutant allele (X^HX^h), the dominant normal allele on the other X chromosome masks the expression of the recessive mutant allele, making her an unaffected carrier.
5. Conclusion:
Because males require only one copy of the recessive gene to show the disease whereas females require two, haemophilia is much more frequently observed in males.