Among symbols $A, B$ and $C$,what does $C$ indicate?

$A$: The possibility of a female becoming a haemophilic is extremely rare.
$R$: Mother of such a female has to be a carrier and the father should be haemophilic.

Which of the following is associated with sex-linked inheritance?

Give scientific reasons: The product of phenylalanine catabolism excreted in the urine in phenylketonuria.

If a father and son are both defective in red-green colour vision,is it likely that the son inherited the trait from his father? Comment.

Consider the following human disorders: $i$. Hemophilia,$ii$. Down's syndrome,$iii$. Cystic fibrosis,$iv$. Color blindness,$v$. Night blindness. Which of these represent Mendelian disorders?