In phenylketonuria:

Pick out the correct statements.
$(1)$ Haemophilia is a sex-linked recessive disease.
$(2)$ Down's syndrome is due to aneuploidy.
$(3)$ Phenylketonuria is an autosomal recessive gene disorder.
$(4)$ Sickle cell anaemia is an $X$-linked recessive gene disorder.

$A$ colourblind man has a colourblind sister but a normal brother. What is the phenotype of his parents?

$A$ colour-blind man marries the daughter of a colour-blind person. Then in their progeny:

Phenylketonuria is caused by the deficiency of which enzyme?

$A$ son of a colourblind father and a normal homozygous mother marries a daughter of a normal father and a heterozygous (carrier) mother. What will be the phenotype of their children?