Discussion
Groups of alleles associated with lactose operon are as follows (in order of dominance for each allelic series) : Repressor - IS-(super repressor), I+ (inducible), I- (constitutive);
Operator - OC (Constitutive, cis dominant), O+ (Inducible, cis dominant)
Structural - Z+, Z-
Which of the following genotypes will produce β galactosidase if lactose is present ?
(I) I+ O+ Z+ Y+
(II) I- OC Z+ Y+
(III) IS OC Z- Y-
(IV) IS O+ Z+ Y+
Answer: A
Only (I) and (II) produce β-galactosidase with lactose present.
- I+ O+ Z+: the normal inducible operon; the inducer releases the repressor, and Z+ is expressed.
- I- OC Z+: there is no functional repressor and the operator cannot bind one anyway, so Z+ is expressed constitutively, including with lactose.
- IS OC Z-: the constitutive operator would allow transcription, but Z- is defective, so no active enzyme is made.
- IS O+ Z+: the super-repressor cannot bind the inducer, so it stays on the normal operator and blocks transcription even with lactose present.
Options including (III) or (IV) are therefore wrong, and '(I) only' misses the constitutive genotype (II).
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