Please can you explain more about selectable markers 

Selectable marker is a gene that select by allowing transformed cells to survive on media contining toxic level of selective agent. The reason why ampR and tetR are called selectable markers is that if we ligate a foreign DNA at the restriction site of tetracycline resistant gene in a vector than they can be selected out from the non- recombinant ones. This is confirmed when the recombinants will grow in ampicillin containing media but not on that containing tetracycline (this happens because recombinant plasmid have lost tetracycline resistance due to insertion of foreign DNA). Now since these ampR and tetR helps in selecting recombinants over non- recombinants they are called selectable markers.