SNPs in the Large Intestine?

I've become increasingly skeptical of the penetrance of SNPs based on a lot of recent research that I've seen. Have you from clinical experience or from studies, have there been anything that validates that pattern as a marker for confirmed genetic impairment in the transporters? That's what I'd like to know. According to a review from 2016, there were studies looking at who is a fructose malabsorber and there was no correlation with polymorphisms in those enzymes. But that doesn't mean that there's not 30 people out there who have their own home-grown mutation that was de novo in them or came from their mother when it popped up out of nowhere. And that person