For selection of recombinants, insertional inactivation of antibiotic resistance   is a marker that has been superceded by insertional inactivation of a marker gene coding for a chromogenic substrate.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

A marker gene helps in differentiating between transformant genes and non-transformant genes. This helps in selecting the suitable recombinants. In case of E-coil; pBR322 is the vector for resistance to antibiotic tetracycline. The insertional inactivation of pBR322 will result in loss of resistance to tetracycline by E.coli. This can be found out by growing the recombinants on two plates; one containing tetracycline and another containing ampicillin. The recombinant will grow in ampicillin but not in tetracycline. A marker gene for chromogenic substrate helps in identifying the recombinant DNA on the basis of gain or loss of colour from the chromogenic substrate. Insertional inactivation of a marker gene coding for a chromogenic substrate will result in no blue colour imparted in the colony. These steps are taken for easy Identification of recombinants from non-recombinants.