What does the given symbol indicate?

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.

Assertion: Phenylketonuria is a recessive hereditary disease caused by the body's failure to oxidize the amino acid phenylalanine to tyrosine due to a defective enzyme.
Reason: It results in the presence of phenylalanine acid in urine.

Sickle-cell anaemia is

The disease 'Haemophilia' is caused due to the lack of:

Which of the following is a sex-linked disease?