For selection of recombinants, insertional inactivation of antibiotic marker has been supercoded by insertional inactivation of a marker gene coding for a chromogenic substrate. Give reasons.

Explanation

In selection of recombinants due to ineactivation of antibiotics, the transformed cells are first plated on the antibiotic plate which has not been insertionaly inactivated (i.e., amplicilin) and incubated overnight for growth of transformation.
For selection of recombinants, these transformation are replica-plated on second antibiotic (say-tetracycline) plate(which got inactivated due to insertion of gene),
Non-recombinants grwo on both the plates (one carrying ampicillin and the other carrying tetracycline) while recombinants will grow only on amplicin and the other carrying labourious and takes more time (two overnight inclubation) as well.
However. if we choose insertional inactivation of a marker that produces colour in the pressure of a chromoganic compound, we can distinguish between the recombinants and non-recombinants of a single medium plate (contaning one antiobiotic and the chromoganic compound) after overnight growth.