How many of the given statements (i-iv) is/are correct?
(i) In transcription, adenosine pairs with uracil.
(ii) Regulation of lac operon by repressor is referred to as positive regulation.
(iii) The human genome has approximately 50,000 genes.
(iv) Haemophilia is a sex-linked recessive disease.

Statement (i) and (iv) are correct.
Regulation of lac operon by repressor is referred to as negative regulation. In negative regulation, a repressor molecule binds to the operator of an operon and terminates transcription. In positive regulation, an activator interacts with the RNA polymerase in the promoter region to initiate transcription. Human genome contains some 20,000 - 25,000 genes billion bases.