Haemophilia is more commonly seen in human males than in human females because:

A man and a woman, who do not show any apparent signs of a certain inherited disease, have seven children (2 daughters and 5 sons). Three of the sons suffer from the given disease but none of the daughters are affected. Which of the following mode of inheritance might be the reason for this disease?

1. Sex-limited recessive

2. Autosomal dominant

3. Sex-linked recessive

4. Sex-linked dominant

A normal woman, whose father was colour-blind is married to a normal man. The sons would be:
1. 50% colour-blind
2. All normal
3. All colour-blind
4. 75 % colour-blind 

Which of the following is the example of a sex-linked disease:

1. AIDS

2. Colour blindness

3. Syphilis

4. Gonorrhoea