A man who is achondroplastic dwarf with normal vission marries a colour-blind woman of normal height. The man's father was six feet tall, and the woman's parents were of average height. Achondroplast dwarf is autosomal dominant, and red - green colour blindness is X - linked recessive. What proportion of their son will be colour-blind and of normal height ?
1. All
2. None
3. Half
4. One out of four