Myotonia congenita is caused by an inherited disorder of a chloride channel in the muscles of the skeleton (skeletal muscle chloride channel 1, CLCN1). Congenital myotonia can be inherited as an autosomal dominant trait (with incomplete penetrance) or a recessive trait, resulting in the varying severity of the condition. In affected goats, the CLCN1 gene contains a missense mutation; the amino acid alanine is replaced with a proline residue. This small change causes the chloride channel in the muscle fibres to have a reduced conductance of chloride ions. This missense mutation occurs in a sequence of seven amino acids that are included in a group of closely related channels including that of humans and rats. This causes a delay in the relaxation of the muscles after the goat has made an involuntary movement. After stimulation, in myotonia congenita there is an increased tendency of the muscle fibers to respond with repetitive action potentials and after discharges. It has been shown that the increased muscle excitability is largely accounted for by the lack of chloride permeability in these fibers. Myotonia congenita is also characterized by a significant increase in the fast isomyosins in each muscle type. The muscle fibers of the myotonic goat were found to be highly (electrically) resistive, corresponding to the blocking of chloride conductance. In a study, normal goat muscle fibers could be made myotonic by blocking the chloride conductance using myotonia inducing drugs, or by substituting in an anion that is unable to pass through a semi-permeable membrane.