[LukyTom (OP)]

Hey guys, hoped i posted in right forum.

I was reading about the environmental effect on phenotype, and how there is a definite link. Firstly I am quite curious about the mechanism of this process; per say, does the environment trigger the genotype, which then triggers a phenotype? I.e. Can the environment switch of a gene? Or is it by some other mechanism that the environment affect the phenotype.

Secondld, if it does, (if it dosent, i am still quite interested to know) what is the molecular mechanism that turns a gene "off" assuming such a theory is scientifically valid.

[Variola]

It kinda depends what you mean on phenotype;- in humans phenotype is often used to describe a disease state, i.e Down's syndrome is a phenotype. But whereas the Down's syndrome phenotype is not considered to be significantly affected by the environment, a diabetic phenotype could be. So it is a bit ambiguous.
However when it comes to life on the smaller scale it is much easier. I can go into the lab, and created a different phenotype of bacteria simply by growing it on a different media or subjecting it to heat shock. The phenotype is essentially the characteristics of that organism expressed at that time.
The environment can most definitely affect the switching on and off of genes. Example, during heat stress bacteria like E.Coli and S. Coelicolor produce different proteins to help them cope with the heat stress, these are genes not usually switched on, but the sensing of heat activates the regulon, and the genes needed are expressed. It can also switch off other genes, for example again during heat stress there is not much point using up energy in growing and dividing, so bacteria downregulate, or switch off genes involved in mitosis. Food plays a part too, E.Coli will use glucose as a food source above everything else, but when that is used up, it activates genes  in the Lac Operon, so it can release enzymes that enable it to use lactate.
Turning genes on and off happens in so many ways it is difficult knowing where to start! In the example above of the Lac Operon, expression of this set of genes is usually off, the promoter region upstream of the gene has a repressor protein bound to it, there would be no point expressing these genes while glucose is abundant. When glucose is used up, the presence of small amounts of lactose in the cell, taken up by the E.Coli signal for the  repressor protein to be knocked off the operon and RNA Polymerase can do it's job of transcribing these proteins.

I hope that helps a bit, I have kinda assumed some knowledge of genetics above, but if you need more explanation I am happy to give more if I can :-)