A key factor in gene flow and environmental impact is surely the nature of the gene itself. If a gene improves the survivability of the genome in whatever environment the genome encounters, i.e. a 'natural' setting or a 'toxic' man-made setting, then that gene is likely to spread and have an impact. If the gene is neutral or slightly negative as far as survivability is concerned, e.g. the production of vitamin A by rice, then the gene is less likely to spread and even if it does is less likely to have any serious impact. Any argument about gene flow and environmental should take this into account.

I would point out that following the above argument adding herbicide resistance genes to plants, which appears to be the most common form of 'GM' in the UK, then applying herbicide is a recipe for disaster. If genes can be transferred naturally to other plants the selective pressure almost guarantees the spread of herbicide resistance wherever herbicide is applied.

Background: Senior Engineer