What indicates the presence of recombinant colonies in the method of…

The correct answer is Option (2) → The colonies remain colourless or white.

In this method the foreign DNA is inserted inside the gene for the enzyme β-galactosidase. The insert breaks the gene, so the enzyme is not made; this is called insertional inactivation. The bacteria are grown with a chromogenic substrate, which turns blue only when the enzyme is present.

Option (1) The colonies turn red. — Incorrect. No red colour is produced in this method. The substrate gives only a blue colour with the active enzyme.

Option (2) The colonies remain colourless or white. — Correct. A recombinant plasmid has the insert, so β-galactosidase is not made. The colonies do not produce any colour and stay white; these are the recombinants.

Option (3) The colonies turn blue. — Incorrect. Blue colonies have a plasmid without an insert. The working enzyme acts on the substrate, so they are non-recombinants.

Option (4) The colonies fluoresce under UV light. — Incorrect. Fluorescence under UV is used to see DNA bands stained with ethidium bromide in gel electrophoresis, not to pick recombinant colonies.

NCERT Extract:
Cloning Vectors – “… In this, a recombinant DNA is inserted within the coding sequence of an enzyme, β-galactosidase. This results into inactivation of the gene for synthesis of this enzyme, which is referred to as insertional inactivation. The presence of a chromogenic substrate gives blue coloured colonies if the plasmid in the bacteria does not have an insert. Presence of insert results into insertional inactivation of the β-galactosidase gene and the colonies do not produce any colour, these are identified as recombinant colonies.”