What are genetically modified crops?
GM crops come in two main types. The first are given a gene that enables them to tolerate herbicides that wipe out all other plants, which allows farmers can spray their crop with a "broad-spectrum" herbicide. The second type are given a gene that lets the plant produce a toxin that kills pests that would usually feed on it, making the GM plant resistant.
Who makes them?
Monsanto produces more than 90% of GM crops worldwide. Companies such as Syngenta, Bayer CropScience, Dow and BASF make the rest.
How do they do it?
One technique uses a natural soil bacterium called agrobacteria tumefaciens, which naturally infect plants and place some of their own DNA into the plant's genes. Scientists replace these genes with those carrying the useful GM trait, and use them to transform plant cells.
Another method is a gene gun, which fires tiny gold particles coated with genes directly into a plant's cells. Both techniques have a high failure rate, so thousands of attempts are needed.
What crops are grown?
The four main commercial crops grown are soya beans, maize, cotton and oilseed rape. Companies are also looking to develop GM versions of other crops, including rice.
Are they grown in the UK?
There are no GM crops grown commercially in Britain, though there are no legal reasons why companies couldn't apply for the relevant licenses to do so in future. Then it would be up to farmers and whether they believe there would be a demand.
What about experimental trials?
There have been hundreds of small-scale outdoor trials of various GM crops, including the government's farm-scale evaluations earlier this decade. BASF is actively growing GM crops outdoors, a trial of blight-resistant potatoes in Cambridge. In May the government granted the University of Leeds permission to run a three-year trial on potatoes that have been engineered to resist eelworm.
Where are they grown in the world?
GM crops are grown by more than 10 million farmers on 252m acres in 22 countries. Spain is the biggest grower in Europe, but there are also significant amounts of crops grown in France, Germany, the Czech Republic, Slovakia and Portugal. Around the world, the US, Argentina, Canada and Brazil plant the most, with various amounts grown in China, Australia, Bulgaria, Colombia, Honduras, India, Mexico, South Africa and Uruguay.
Are we eating GM in the UK?
Very few GM products are on sale in Britain. Most supermarkets and food companies have imposed bans on imported GM ingredients such as soya and rape seed oil in processed food, though small quantities may slip in because of contamination earlier in the manufacturing process. There are a handful of clearly labelled GM-containing foods on the shelves, such as bacon-flavoured soya chunks. Many dairy products, eggs and meat could have come from animals fed imported GM crops, but they are not considered to be GM produce.
Why the controversy?
Some people object to GM technology because it can be used to create unnatural organisms. For example, a plant can be modified with genes from another species of plant, or even an animal. Another concern is that genes used to modify crops could escape into wild plants, creating "superweeds" that are highly resistant to pests, or alter plants in other ways that might cause damage to the environment. Some worry that GM crops themselves might prove to be harmful to either wildlife or the people who eat the crops.
There are also concerns about the accidental contamination of organic produce, which markets itself as GM-free.
What about the advantages?
The companies argue that GM crops can reduce weedkiller use, and improve yields by offering useful resistance to pests and poor growing conditions caused by drought or salinity.
What about their impact on the developing world?
The companies point out there has been a large take-up of GM crops in poorer countries, and regularly claim that GM crops address many of the problems that blight farmers there. Anti-GM campaigners point out that most GM crops are grown for animal food, and massive amounts grown in countries such as Argentina are exported, sometimes at the expense of local food production. They also worry about the control of seeds and agricultural techniques by a handful of multinational companies.
What other types of GM crops are there?
The next generation of GM crops produce drugs and vaccines, called pharming. GM tobacco plants, for example, can churn out a topical medicine against HIV infection. Advocates say the use of plants opens up cheap routes to new medicines. Critics worry about their use of food crops because it might lead to contamination of food supplies.