Degenerative myelopathy (DM) in dogs is a progressive adult-onset spinal cord degeneration associated primarily with SOD1 variants, documented as autosomal recessive with incomplete penetrance (OMIA:000263-9615).
The most-discussed SOD1 change in that record is c.118G>A in exon 2. Dogs with two copies are at elevated risk, but some homozygous dogs never show clinical signs, which is why OMIA records incomplete penetrance (and points at other loci and modifiers in the literature). A second SOD1 change (c.52A>T) is discussed mainly in Bernese Mountain Dogs.
Sniff observes the condition across 138 of 266 breeds tested in the atlas layer, with measured carrier frequencies drawn from 242,641 dogs (Donner 2023). Carrier frequency describes variant prevalence in tested cohorts, not disease incidence for your dog. Being a carrier is not the same as being affected.
Canine DM is studied as a natural model of SOD1-related motor neuron disease in people; it is not the same as a human ALS diagnosis. For full OMIA prose, breed carrier tables, and citations, use the disease page and the OMIA record.