Final answer:
The mutation in the gene that codes for the ion channel is a loss-of-function mutation, resulting in a constantly open channel regardless of ligand presence.
Step-by-step explanation:
The mutation in the gene that codes for the ion channel is a loss-of-function mutation. This type of mutation results in a non-functional or defective protein. In this case, the mutation in the gene coding for the wild type ion channel leads to a mutation that prevents the ligand from properly binding to and closing the channel, resulting in a constantly open channel regardless of ligand presence.