This is how our neurologist gave it to our family. If not, then the chances are probably zero. The two most common forms of familial als are Autosomal Dominant Gene transmission. The second form is a recessive gene where two recessive genes must come together to get the disease. The chances of getting the recessive form are pretty slim when you take into account the need for both parents to be carriers of the gene and pass it on to the children.
Family history is pretty important.
I haven't seen a great site for this kind of information, sorry. Maybe somebody else has.