Mashi, definitely genetics in there... but like with many things genetic (like cancers for example) there also tends to be the trigger factor. ie, My sister was colicy, but has always been the "dairy queen" & she eats heaps of dairy & none of her children have issues & only one of mine has an issue & that is the child that is the "mini me"... where as DS2 is very much DH's side & he has no issues with Dairy, even though one cousin on DH's side has big Dairy issues, but so does his Mum (not related to DH), so guessing the fact DS1 & nephew both have issues isn't related to DH's family but to mine & SIL's.
If there is a gentic/exposure link (which is probable) then having the "genetics" & the "exposure" wouldn't mean it was necessarily higher, as it would depend on the prevelance of a "gene" in society & what impact the trigger was... ie if only 5% of the population have the gene, but all of them express a reaction, then only 5% would continue to express, but if it was linked to a gene that was say in 80% but only expressed on 20% of triggers then you'd expect the rate to get higher.