A boy is colour blind, in his two sisters one is colour blind and one is carrier $(normal)$. Then who is colour blind in his family

Person whose father is colourblind marries a lady whose mother is daughter of a colourblind man. Their children will be

Sickle -cell anaemia has not been eliminated from the African population because it

Red green colour blindness appears due to

Which of the following most appropriately describes haemophilia?