In Mendelian inheritance, recessive phenotype is  expressed when wild type allele is mutated to form a

 A- less functional enzyme

B- non functional enzyme

C-no  enzyme product

D- functional equivalent to original enzyme

NCERT XII, P.NO: 76   

The normal allele (Wild type - dominant) produces the normal enzyme that is needed for the transformation of a substrate S. When the allele modified it could  produce –

(i) the less functional enzyme, or (ii) a non-functional enzyme or
(iii) no enzyme at all or (iv). functional equivalent enzyme .

In case (ii) and( iii), the gene product is not formed so the substrate is not converted to product, which  leads to expression of recessive phenotype.