"GM" stands for genetic modification. A gene is an instruction and each of our cells contains tens of thousands of these instructions. In humans, these instructions work together to determine everything from our eye colour to our risk of heart disease. The reason we all have slightly different characteristics, even from our brothers and sisters, is that before we are born our parents’ genes get shuffled about at random.

Exactly the same principles apply to plants. If you're a gardener, you might save seed from a favourite plant, hoping you'll get another plant exactly the same. But, because genes get shuffled about, you might get something that looks rather different. It's still the same kind of plant but it's bigger or smaller or a slightly different colour.

For thousands of years farmers have selected plants with the characteristics they want, such as extra seeds in a pod or the ability to survive in the cold. By crossing the best plants they hoped to produce even better varieties. But this approach is a bit like playing a fruit machine: you hit the jackpot only very occasionally. So since the 1950s, plant scientists have lent a hand.

Deliberately exposing seeds to radiation, for instance, increases the chance that one of them might produce a more useful plant. The barley variety Golden Promise was produced in this way and has been growing in Britain for 30 years.

Unlike these earlier methods, GM techniques allow specific genes to be copied into a plant. Because the scientists know a great deal about the genes they're working with, it's easier for them to 'track' the genes, understand their effects and eliminate unwanted side effects long before the plants are used in field trials or grown commercially.

GM allows chosen individual genes to be transferred from one organism into another, including genes between non-related species. Such methods can be used to create GM crop plants. The technology is also sometimes called “modern biotechnology”, “gene technology”, “recombinant DNA technology” or “genetic engineering”.

The actual transfer of genes into the selected organism (a plant for example) always takes place in a laboratory under carefully controlled conditions. Genetically modified plants can later be trialled in a special glasshouse or in fields under regulatory oversight, before being grown commercially, using systems long in use for testing and evaluating new plant varieties.

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