A husband and wife have normal vision but fathers of both of them were colour blind. Probability of their first daughter to be colour blind is

Recessive characters are expressed

Which one is the incorrect statement with regard to the importance of pedigree analysis?

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 dominant gene disorder

$IV.$ Phenylketonuria is an autosomal recessive gene disorder

$V.$ Sickle cell anaemia is an $X$-linked recessive gene disorder

The amino acid substituted in sickle cell anaemia is