The disease may not be genetic. Only a small percentage is. My dad has ALS and in his case is not genetic (lucky me). The disease starts and progresses in different paces for different people - my dad has it for almost 7 years and still talks and eats like before. For him, the first thing to go was his arms; now his legs are almost paralyzed as well.

In his case, it probably is associated with the fact that he worked with agrochemicals his whole life, in a time when regulations and PPE where much loosier. He tells stories of taking baths of substances while his dad worked in orchards.