Hemophilia in humans is controlled by the

1. Analyze the question The question asks how hemophilia, a blood-clotting disorder, is genetically controlled.

2. Explain the concept Hemophilia is a sex-linked (X-linked) recessive disorder. The genes for the clotting factors (Factor VIII for hemophilia A, Factor IX for hemophilia B) are on the X chromosome.

• Males (XY) have only one X chromosome. If it carries the hemophilia allele (X^h), they have the disease.
• Females (XX) need two copies of the recessive allele (X^hX^h) to be affected. With one copy (X^HX^h) they are carriers and are usually normal.

3. Step-by-step solution

• A. Blood group: Blood groups (A, B, AB, O) are controlled by the ABO gene system and have nothing to do with clotting factor genes. Wrong.
• B. Dominant gene: If it were dominant, a single copy would cause the disease, and carriers would not exist. Hemophilia has carrier females, so it is not dominant. Wrong.
• C. Rhesus factor: This is an antigen on red blood cells (Rh+ or Rh−), relevant to blood transfusion and pregnancy compatibility, not clotting. Wrong.
• D. Recessive gene: Correct. The allele is recessive and carried on the X chromosome.

4. Answer The correct option is D. recessive gene.

5. Tip Remember "Hemophilia = X-linked recessive". Other sex-linked recessive conditions you will often see in JAMB are colour blindness and hemophilia. Affected males are more common because they have only one X chromosome. A carrier mother (X^HX^h) and normal father (X^HY) will have 50% of their sons affected and 50% of their daughters as carriers.