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Genetic modification, also known as genetic engineering, involves altering the genetic make-up of an organism by inserting genes or portions of genes from another plant, animal or micro-organisms using techniques of biological engineering, rather than traditional plant or animal breeding which by definition involve blending two or more whole genomes. In UK legislation a GMO is defined as an organism, with the exception of human beings, in which 'the genetic material has been altered in a way that does not occur naturally by mating and/or natural recombination'. All living organisms contain genetic material in the form of DNA and/or (in the case of some viruses) RNA, and transfer of genetic material between different organisms can alter the organism's characteristics (phenotype). The genetic material provides the blueprint for the development of the phenotype. Humans have been modifying the genetic make-up of organisms, particularly those we cultivate or domesticate, for many years, through intentional and unintentional selective breeding programmes. Genetic engineering differs from traditional breeding in many ways, but one of the most controversial is that genes from very distantly related organism can in theory be inserted into a plant or animal (or micro-organism) to alter a wide variety of characteristics, from herbicide resistance to vitamin A content.

It may seem strange to insert genes from one organism into another, but in fact this often happens in nature itself, with viral genes being incorporated into the genomes of higher plants, for example. The genome of any organism is an incredibly dynamic system and is constantly changing, through natural selection and many other modifying processes. The biotechnological techniques used in genetic modification involve processes of identifying genes of interest, tagging them in order to keep track of them once they are transferred, and then the bombarding of nuclei of the plant or animal to be modified with tiny bits of this tagged DNA. With current technology this process is sometimes rather 'hit and miss'. The potential benefits and risks of this technology are widely debated in the public media, often from entrenched and emotional viewpoints. One balanced view, expressed in the scientific journal Nature, is that 'evolving techniques for redesigning organism have enormous potential but they must be matched with equally sophisticated methods for evaluating their benefits and risks'.

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