Question

Haemophilia is a genetic disorder of coagulation seen only in males. It is transmitted as

• A. X-linked dominant
• B. Y-linked dominant
• C. X-linked recessive
• D. Autosomal recessive

Hint:

Work out what a male needs on his single X chromosome to be affected, compared to a female.

Answer 1

The Correct Option is C

Step 1: Understanding the Question.
Haemophilia is a bleeding disorder that shows up almost only in boys and men. The question asks how the faulty gene passes from parents to children to give this pattern.

Step 2: Key Concept or Approach.
The gene for clotting factor VIII (haemophilia A) or factor IX (haemophilia B) sits on the X chromosome. A trait is called X-linked recessive when one faulty copy on the X chromosome causes disease in a male, but a female needs faulty copies on both her X chromosomes to be affected.

Step 3: Detailed Explanation.
A male has one X and one Y chromosome, so whatever gene sits on his single X shows up directly, since there is no second X to cover for it. If that X carries the faulty clotting gene, he has haemophilia.
A female has two X chromosomes. If she carries the faulty gene on only one of them, her other normal X makes enough clotting factor, so she stays healthy and becomes a silent carrier. She only gets the disease if both her X chromosomes carry the fault, which is rare.
This is why haemophilia is seen almost only in males, and why an unaffected carrier mother can pass the gene to her sons.
Now check the other options. "X-linked dominant" is wrong, because a dominant X-linked gene would cause disease even in carrier females, which does not match what we see with haemophilia. "Y-linked dominant" is wrong, because the Y chromosome does not carry the clotting factor genes, and a Y-linked trait would pass straight from father to every son with no carrier mothers at all. "Autosomal recessive" is wrong, because genes on the non-sex chromosomes affect both males and females equally, but haemophilia is seen almost only in males.

Step 4: Final Answer.
Haemophilia is transmitted as an X-linked recessive disorder.