Hello, I’ll start off with a few POVs first.
To me being smart or clever is different from being intelligent. one example are, for example some murderers. They have a bad record (yes generalizing a bit, not meaning all…) in schools or high schools (if they made it that far), yet they can plan almost perfect murders, or thieves that get away with it uncaught. They may not be smart, i.e. know SqRt(3289326579415), but they are intelligent, to know what will give them away, what won’t and so on… moving on to the forum talk as well…
Firstly, I have never seen a more irrelevant statement on that thread. First of all, being a lawyer doesn’t mean one knows biology, maths, geography and so on perfectly, it means they know literature (syntax, tone, way of speech etc. as speeches and talking is an art if done properly) and, of course, law. I guess if I would ask a lawyer the SqRt(3289326579415) they wouldn’t know to answer… doesn’t make them stupid. It makes them lawyers because that is their job and what they know: the LAW. Ergo, their child might have only Cs and Ds, but he might as well become a brilliant lawyer on the long run.
Moving on… while I do believe genetix plays a role in this, and I say it in a loose way… hmm, let me exemplify. Think of the human body as a PC - the brain would be a combo between the RAM and the HD.HD ould be the memory on the long run and the RAM the memory you use now that retains current information ready to be registered later. The naalogy may not be 100% accurate, but follow me here 
You’ll see my point. The more RAM a PC has, the more memory it will use and won’t need a swap file as help (let’s say ntoebooks, agendas and whatever else we use to write thigns down to not forget). having a bigger HD means we can hold more things and use later. So moving on to my point, I do assume (or guess) that there is a gene controlling the size of our HDs and RAM. So I assume what they did with the Doogies was basically “upgrading” their HDs and RAM. Nothing more. This might explain somehow, how a person can use more (or less) brain power than others. I once heard (don’t remember exactly, and dont’ know if it’s true) that Einstein used more o fhis brain than the average smart person (if someone can confirm and give some numbers I’d appreciate it). So perhaps this is an explanation.
To move on to the topic. I don’t think that intelligence is either genetic or environmental. For example, here, in the countryside the education is below a decent level. So they msotly learn thigns they’ll need, like basic arithmetics and simple stuff. They won’t go as far as learning anatomy or chemistry or linear programming, vectorial spaces and the likes. But it’s not cos they are stupid, is because they have limited possibilities. Well, my cousin was, unfortunately for her, born there. Her dad is a worker and her mom a farmer. So not the most Einstein-ous persons in the world. Her brothers are not that smart either. They are slightly below average but they’re not stupid (as I said, within proper circumstances they’d be quite smart). Anyway, moving on… she came here, and as you can imagine the competition here was 15 times more than she exected. Well she got in a uni with the maximum grade, 10 out of 10 and graduated with 10 out of 10, even though her basic education was done there, with which knowledge she’d normally wouldn’t have got more than 4. So, in my opinion it all depends on the person.
Another problem is people misinterpreting “stupidity”. Just because someone doesn’t know geography, doesn’t mean they are stupid - it means they don’t know geography because they didn’t bother to learn it. But they could know something else (variation of the lawyer example). In other words, such a person could just as well learn geography as easy/good as any other person if they’d want to (variation of my cousins example).
I think being stupid is your incapabilitiy of learning in general. Or having a bad or wrong mentality. The reason I say this is because, let’s take me, for example. I tried art, design, graphics and everything related… I really have NO TALENT at it. Sure you might say “skill comes out of practice”. But if I have 0% talent, what skill could possibly come out of it, if my hand can’t draw a straight line ? However I am very good in programming and can understand math statements on first or second glances even if I didn’t study that certain topic. On the other hand, it doesn’t make me a math/IT genius either, does it ? 
Talent playes a very important role. I fooled around with music a bit and discovered I have an amazing musical ear (I pick up around 65%-70% of the notes accurately and can reproduce them). However since I didn’t study music I don’t have the skill to reproduce whole songs (or instrument music lines). Although because I have talent, I could easily learn it and gain skill from learning it. See my point ?
I for one don’t consider myself smart, simply because to me, being smart is knowing a lot of things - but knowledge related, things like you’d use in “Jeopardy” or “Do you want to be a Millionaire ?” or other similar shows. Intelligence on the other hand is something I see as being able to learn and understand things very easily. I do think I am very intelligent though.
As for stupidty used as a loose term - I think everybody has their “stupid” days. Those days when you do things and the next day you tell yourself “what the heck was I thinking doing/saying that ?!”. Doesn’t make you a stupid person though… but there are these people who ask you to explain something to them. You explain - they don’t understand. You rephrase - they don’t understand. You use simpler terms - nothing. Then you really lose your mind and ask “Are you stupid or something ?!”. That is stupidity I guess (or one form of it).
BTW: mentalsoup.com/mentalsoup/basic.htm (hope it wasn’t posted before). The text itself is harsh, but the concept is interesting and does make you wonder. While I don’t agree to it fully, the one assembling the concept initially did have a point.
So yeah I do agree with you basically
Anyway, enough rant. Cheers.
P.S. Happy Easter bunny lovers