How does $\beta$-galactosidase coding sequence act as a selectable marker ? Explain. Why is it a preferred selectable marker to antibiotic resistance genes?

(i) Presence of a chromogenic substrate gives blue color, if the plasmid in the bacteria does not have an insert (non-recombinants).br>(ii) With the insert do not produce any color, or recombinant colonies.
(iii) Selection of recombinants due to inactivation of antibiotics, requires simultaneous plating on two plates having different antibiotics/processes is more cumbersome.
The selection of recombinants due to the inactivation of antibiotics is a difficult process and requires simultaneous plating on two plates having different antibiotics. Thus, to differentiate between the non-recombinants and recombinants The enzyme $\beta$ - galactosidase is preferred as a selectable marker as it allows the person to differentiate the non-recombinants from recombinants. in this case when the recombinant DNA is inserted into the coding sequence of the enzyme $\beta$-galactosidase. The insertion leads to the inactivation of the enzyme and hence in the presence of a chromogenic substance the recombinants do not give colored colonies, while the nonrecombinants produce blue colonies. Thus no separate plating is required.