These crossings are usually between individuals of the same, or closely related, species. The gene pool available for use, in traditional crossing, is thus limited to those genes present in individuals which can be induced to breed using natural crossing methods. In addition to the limited gene pool, because groups of genetic characteristics are inherited together, when a crop plant is selected for the acquisition of a particular desirable trait e.g larger seeds, large numbers of other genes, some of which may be undesirable, will also be selected. These genes need to be removed, as far as possible, usually by back-crossing. The nature of crop growing seasons, or animal generation times, means that the process is slow.
Genetic engineering extends the available pool of genes and speeds up the process.
Genetic engineering depends upon the ability to identify and manipulate specific genes. There are around 100,000 genes in a mammal. Genetic engineering can involve moving genes both within or between species. It is the movement of genes between species which is of the greatest concern. Organisms modified in this way, are referred to as being transgenic or genetically modified or simply as GMOs.