Haemophilia is a hereditary bleeding disorder linked to the X chromosome. In typical human inheritance patterns, it is usually carried by which sex and most often expressed in which sex?

Introduction / Context:
Haemophilia is a genetic disorder in which the blood does not clot properly due to deficiency of certain clotting factors. It is classically described as a sex linked disease associated with the X chromosome. Understanding how it is inherited helps explain why it frequently appears in males while often being passed on by females who do not show severe symptoms. This question focuses on that characteristic pattern of carriage and expression.

Given Data / Assumptions:

• Haemophilia is mentioned as hereditary and sex linked.
• The options describe different combinations of carrier and affected sexes.
• We assume standard human XY sex determination with one X and one Y chromosome in males and two X chromosomes in females.

Concept / Approach:
In X linked recessive disorders such as classical haemophilia, the defective gene is located on the X chromosome. A female has two X chromosomes, so if she carries one normal and one mutant allele, the normal allele usually compensates and she is typically a carrier with mild or no symptoms. A male has only one X chromosome and one Y chromosome. If his single X chromosome carries the mutant allele, he has no second normal copy to mask its effect, so the disease is expressed. As a result, haemophilia often appears in males whose mothers are carriers of the defective gene.

Step-by-Step Solution:
Step 1: Recognise that haemophilia is an X linked recessive disorder. Step 2: Recall that females have two X chromosomes and can carry one defective allele without necessarily showing the disease, making them carriers. Step 3: Recall that males have one X and one Y chromosome, so a single defective allele on their only X chromosome will cause the condition. Step 4: Conclude that the classic pattern is that the disorder is carried by females and expressed mainly in males, so that option should be selected.

Verification / Alternative check:
Pedigree charts in genetics textbooks often show haemophilia running through families where unaffected carrier mothers have affected sons. Historical cases, such as haemophilia in European royal families, follow this pattern. When such pedigrees are analysed, the trait is seen passing from carrier mothers to their sons and sometimes through daughters to the next generation, which agrees with the concept of female carriers and male expression.

Why Other Options Are Wrong:

• Carried by males and expressed mainly in males: Males who carry the mutant allele generally express the disease, but they do not usually act as silent carriers in the same way as heterozygous females.
• Carried by females and expressed mainly in females: This pattern would be expected for a dominant trait, not a recessive X linked trait like classical haemophilia.
• Carried by males and expressed mainly in females: This does not match the X linked recessive inheritance pattern observed in haemophilia.

Common Pitfalls:
A frequent source of confusion is mixing up autosomal and X linked inheritance. With X linked recessive traits, the key is to remember that males have only one X chromosome, so they are much more likely to show the disease if they inherit a defective allele. Another pitfall is thinking that males can be carriers in the same way as females. In classical teaching, carrier status for X linked recessive disorders usually refers to heterozygous females who have one normal and one mutant allele.

Final Answer:
Haemophilia is usually carried by females and expressed mainly in males due to its X linked recessive mode of inheritance.