An attribute of growth behavior of eight bacteriophage mutants (1 to 8) is investigated in experiments that establish coinfection by pairs of mutants. The experiments determine whether the mutants complement one another  or fail to complement  These eight mutants are known to result from point mutation. The results of the complementation tests are shown below.

A new mutation, designated 9, fails to complement mutants 1, 3, 5, 7, and 8. Wild-type recombinants form between mutant 9 and mutations 3, 5, and 8; however, no wild-type recombinants form between mutant 9 and mutations 1 and 7. What kind of mutation is mutant 9? Explain your reasoning.