The following constructs, A or B, have been introduced into two different cultures of eukaryotic cells.

Cre recombinase can act upon loxP–loxP and loxN–loxN sites. The following outcomes have been predicted on the types of fluorescence emitted by the cells upon induction of Cre-recombinase:
A. The cells transfected with construct A will yield three types of cells: each type expressing either red or green or yellow fluorescence.
B. The cells transfected with construct B will yield three types of cells: each type expressing either red or green or yellow fluorescence.
C. The cells transfected with construct A will yield two types of cells: each type expressing either red or green fluorescence.
D. The cells transfected with construct B will yield two types of fluorescent cells; each type expressing either red or yellow fluorescence.
Which one of the following options represents the combination of all correct statements?

Correct option is A

Correct Answer:
(a) A and D
Explanation:
In Construct A, the reporter genes (RFP, GFP, YFP) are separated by distinct recombination sites (loxP and loxN) in such a way that Cre-mediated recombination can excise different segments independently, leading to mutually exclusive expression of RFP, GFP, or YFP in different cells. Hence, three fluorescent cell types are produced (A correct).
In Construct B, recombination leads to excision outcomes that allow expression of either RFP or YFP, but not GFP as a stable end product. Thus, only two fluorescent cell types are generated (D correct).
Information Booster :
· Cre–lox systems are widely used for conditional gene expression.
· Different lox variants (loxP, loxN) recombine only with identical sites.
· Arrangement and orientation of lox sites determine excision outcomes.
· Such constructs enable lineage tracing and stochastic reporter activation.
Additional Information (Incorrect Options):
Statement B: Incorrect because construct B cannot generate stable GFP-only expression.
Statement C: Incorrect since construct A can generate three, not two, fluorescent outcomes.
Hence, options including B or C are incorrect.