The blue and white selectable markers have been developed which differentiate recombinant colonies from non recombinant colonies on the basis of their ability to produce colour in the presence of a chromogenic substrate.
Given below are two statements about this method:

Statement I : The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies.
Statement II : The colonies without blue colour have DNA insert in the plasmid and are identified as recombinant colonies.
In the light of the above statements, choose the most appropriate answer from the options given below:

Blue‑white screening is a common technique used to identify recombinant colonies that have taken up a plasmid containing a DNA insert.

The plasmid vector carries the lacZ gene fragment that encodes the α‑peptide of β‑galactosidase. When the vector is transformed into a host strain that provides the complementary ω‑peptide, functional β‑galactosidase is reconstituted and can hydrolyze the chromogenic substrate X‑gal, producing a blue colour.

If a foreign DNA fragment is ligated into the multiple‑cloning site (MCS) within the lacZ α‑fragment, the gene is disrupted. The disrupted lacZ cannot produce functional β‑galactosidase, so the colonies remain white (no blue colour) on X‑gal plates.

Therefore:

• Statement I: “The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies.” This is incorrect because blue colonies retain an intact lacZ gene and lack the insert.


• Statement II: “The colonies without blue colour have DNA insert in the plasmid and are identified as recombinant colonies.” This is correct because white (non‑blue) colonies result from disruption of lacZ by the inserted DNA, indicating successful recombination.

Consequently, the most appropriate answer is that Statement I is incorrect but Statement II is correct.