NEW AND IMPROVED ANIMALS
For thousands of years people have improved crop plants and domestic animals by selective breeding, mostly at a trial-and-error level. Woollier sheep and smarter sheep dogs have both been improved through many generations of selective breeding. Obviously, the more we know about genetics, the faster and more effectively we can improve our crops and livestock. Today it is possible to alter plants, animals, and even humans by genetic engineering. Most early experiments in animal transgenics were done with mice, but many larger animals have now been engineered, including livestock such as sheep and goats, pets such as cats and dogs, and even monkeys. In a transgenic animal every cell carries new genetic information. In other words, novel genetic information is introduced into the germline, not merely into some somatic cells as in gene therapy (discussed in Chapter 17 ). Consequently, the novel genes in a transgenic animal are passed on to its descendants. This novel genetic information generally consists of genes transferred from other organisms and so referred to as transgenes . They may be derived from animals of the same species, from distantly related animals, or even from unrelated organisms such as plants, fungi, or bacteria. The transgenes are themselves often engineered before being inserted into the host animal. The most frequent alteration is to place the transgene under control of a more convenient promoter. This may mean a stronger promoter or a promoter designed to express the transgene under specific conditions. These assorted manipulations have been dealt with in previous chapters. Here we will consider techniques to create transgenic animals and some applications of this technology.