In: Biology
Please correct if they are wrong
| 
 Genotype  | 
 ?-galactosidase  | 
 Permease  | 
 Outcome  | 
||
| 
 Lactose  | 
 No lactose  | 
 Lactose  | 
 No lactose  | 
||
| 
 I+P+O+Z-Y-/ I+P+O+Z-Y-  | 
 +  | 
 -  | 
 +  | 
 -  | 
 lac+  | 
| 
 ISP+OCZ+Y-/ I+P-OCZ-Y+  | 
 -  | 
 -  | 
 -  | 
 -  | 
 lac-  | 
| 
 I+P+O+Z+/ I+P+O+Z+  | 
 +  | 
 -  | 
 -  | 
 -  | 
 lac+  | 
Lac operon is an inducible operon which means that the cistron undergoes transcription only when lactose is prsent in the medium. This is because the lac operon encodes for lactose-fermentation enzymes, hence presence of lactose in the cell/medium will prompt the cells to induce the operon. The lac operon contains a cistron carrying three genes. The lac Z encodes for beta galactosidase, lac Y encodes for permease and lac A encodes for transacetylase. All of these three genes need to be present and expressed for a successful lac operon transcription.
Thus, based upon these information, the answers can be found as below:
Case 1: No expression (the genotype is both negative for lacZ and Y, hence no transcription will take place)
Case 2: No expression (the expression of lac operon is absent in the absence of lactose in the medium)
Case 3: No expression (Since the lac Y gene is altogether absent from the cistron, no expression of cistron takes place)