Pick out the correct statements.

I. Haemophilia is a sex-linked recessive disease

II. Down's syndrome is due to aneuploidy.

III. Phenylketonuria is an autosomal recessive gene disorder

IV. Sickle cell anaemia is an x - linked recessive gene disorder

(1) II and IV are correct

(2) I, III and IV are correct

(3) I, II and III are correct

(4) I and IV are correct

If a colourblind man marries a woman who is homozygous for normal colour vision, the probability of their son being colourblind is:

1. 0

2. 0.5

3. 0. 75

4. 1