Actually the environment can change eye colour - although rare - some drugs can do just that. It is more usual than not for the environment to have a role in inherited conditions. Especially something as complex as autism which involves many systems.
I've had lots of triggers for autism as well, but don't have the genetic makeup for that to be a problem.
Downs Syndrome is completely different- it's arises from non-disjunction - it's not about expression of genes, and the environment can affect the expression of various 'symptoms' of DS as well.
Of course there will be cases where the environment has played a minor role. You can see it in families where the condition runs through the family- with uncles who refuse to talk to anyone, cousins who collect pictures of telegraph poles and the this generation diagnosed with AS. That's one subgroup who may have a totally different condition biologically than other types of autism.
But when you have a family history like my boys, with no autism anywhere in either family (and I have 22 first cousins, my family is huge) but lots and lots of autoimmune and atopic conditions (MS, type 1 diabetes - lots of type 1 diabetes, severe eczema, psoriasis running right through the family generation after generation) and your child regresses following a viral illness (accepted by the pros - it's in the dx letter) then it does begin to look as if the most likely genetic influence is something to do with the immune system.
In 2006 some gastroenterologists published a paper describing a model for the development of autoimmune conditions (mentioning type 1 diabetes and MS) that is very very similar to various autism researcher's models for the development of some cases of autism. It was that you have a leaky gut, then environmental agents pass through and trigger the autoimmune condition. I suspect its a model that may function quite well at some level in our family. Certainly applying it in our family seemed to stop ds3's disordered development its tracks resuming normal service.
Of course it won't apply to every child. But looking at an indvidual child's family history can give you a hunch of the sorts of things that might be going on.