PRACTICAL REASONS FOR CLONING ANIMALS
Why clone sheep? Aside from showing that whole animals can be cloned, there are practical reasons. For millennia, humans have bred farm animals in attempts to improve them. Genetic duplication allows an improved animal to be widely distributed relatively quickly.
In addition, a flock or herd of genetically identical animals will give wool, milk, eggs, or meat of a more standardized quality. Conversely, genetically identical animals will all be susceptible to the same infections, and epidemics will spread faster and further.
Although cloning produces identical animals, the technique may paradoxically help protect genetic diversity. For example, in New Zealand, the last surviving cow of a rare breed was successfully cloned. Thus, cloning allows us to genetically rescue rare breeds of animals or endangered species but avoids mixing their genes with outsiders as would happen in crossbreeding. Table 15.1, which lists animals cloned to date, includes three rarities, the gaur, mouflon, and banteng.
The most important use of animal cloning is in combination with transgenics. Previously created transgenic animals may be cloned for speedier distribution of the product. However, it is also possible to introduce transgenic DNA during the cloning process. The Roslin Institute, where Dolly was born, has since cloned sheep carrying the gene for human factor IX. The transgene was inserted into the nuclear donor cells while in culture. A variety of other transgenic animals carrying pharmaceutically important proteins have also been cloned.
The reason why Dolly was cloned using a mammary gland cell should now be apparent. If a foreign protein is expressed in this tissue, it will be secreted into the milk and be easy to harvest commercially. Once a good transgenic cell line has been established in culture, nuclear transplantation can be used to generate several genetically identical animals for production purposes. Such transgenic cells can be stored over the long term by freezing in liquid nitrogen and have been referred to as protoanimals.