It does not work like that. Everyone has two copies of the gene. It is random which copy gets passed on to an off spring so it is 50 /50 whether we got the defective one. We were unlucky.
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I guess my point is that instead of being 'unlucky' there was (to me) or at least could have been some other unknown factor that caused all of your mother's children to inherit the gene.
Such as, say, a specific unknown gene (not the ALS gene) or make-up of genes that your father had that influenced the genetic make-up of your mother's children which caused them all to inherit this gene from your mother, or maybe, say, your father's genes were not so overpowering so that your mother's children inherited more of her genetic make-up than your father's.
Kind of like, some children look exactly like their father, some look exactly like their mother, and some are kind of a combination... or even have little family resemblance at all.
Or perhaps a combination of other un-known genes in her line that made her children much more prone than others who simply have the one specific gene.
Or perhaps an environmental factor.. ..
Any of these things could alter the risk thus altering the 'laws'.
Perhaps your risk was actually 90% based on your mother's gene and whatever other factor could have influenced your genetic make-up.
Without this information I would not think that we could simply say 'fate', or 'unlucky' or 'random'.