An example of a disease linked to the sex-chromosome is

Step 1: Analyze the question The question asks for a disease that is sex-linked, meaning the gene responsible is located on a sex chromosome (usually the X chromosome), not a disease spread by contact or infection.

Step 2: Explain the concept Humans have 23 pairs of chromosomes. One pair determines sex: females are XX and males are XY. Some genes lie on the X chromosome and have no matching gene on the Y chromosome. This produces sex-linked inheritance.

Because males have only one X chromosome, a single faulty recessive gene on it will show up in a male. Females need two faulty copies to show the condition. This is why sex-linked disorders are more common in males.

Common examples:

• Colour blindness (red-green)
• Haemophilia
• Duchenne muscular dystrophy

Step 3: Examine each option

• A. Colour blindness: Caused by a recessive gene on the X chromosome. It is sex-linked. ✔
• B. Gonorrhea: A sexually transmitted infection caused by the bacterium Neisseria gonorrhoeae. It is not genetic.
• C. Night blindness: Usually caused by a lack of vitamin A, which is a deficiency disease.
• D. Syphilis: A sexually transmitted infection caused by the bacterium Treponema pallidum. It is not genetic.
• E. Tuberculosis: A bacterial infection caused by Mycobacterium tuberculosis. It is not genetic.

Step 4: Answer The correct answer is A. colour blindness.

💡 Exam tip: Do not confuse "sex-linked" with "sexually transmitted." Gonorrhea and syphilis are sexually transmitted infections (STIs), while colour blindness and haemophilia are sex-linked genetic conditions. Remember: "Colour and Clotting" (colour blindness and haemophilia) are the classic X-linked examples in NECO, WASSCE and JAMB.