There are various definitions for the term transgenic animal. The transgenic animal is an animal in which there has been a deliberate modification of its genome, the genetic makeup of an organism responsible for inherited characteristics. The nucleus of all cells in every living organism contains genes made up of DNA. These genes store information that regulates how our bodies form and function. Genes can be altered artificially, so that some characteristics of an animal are changed. For example, an embryo can have an extra, functioning gene from another source artificially introduced into it, or a gene introduced which can knock out the functioning of another particular gene in the embryo. Animals that have their DNA manipulated in this way are knows as transgenic animals. Transgenic animals first emerged during the mid-
1980s and then this work opened the way for applications of transgenesis in animals and has resulted in efforts along four lines.
1. Production of pharmaceuticals expressed in milk,
2. Production of animals as organ donors for xenotransplants,
3. Creation of models for study of human disease, and
4. Development of livestock suitable for use in agriculture.
The goal of gene transfer should be to enhance the quality of life for humans or the livestock being developed, or to significantly benefit resource conservation. Transgenesis offers the opportunity to increase performance of agricultural animals through improved feed intake, enhanced metabolism, better feed conversion, or reduced pathogen load; create novel or enhanced food and fibre products, permitting diversification of agricultural products; and reduce susceptibility to diseases, thereby improving food quality and security, decreasing antibiotic use, and enhancing resource conservation. Medicines required to treat certain human diseases can contain biological products, but such products are often expensive to make. Transgenic animals that produce useful biological products can be created by the introduction of the portion of DNA, which codes for a particular product. For example, large amounts of a human protein (á-1-antitrypsin) are used to treat a life-threatening condition called emphysema. Transgenic sheep were developed which make the protein in their milk in larger quantities than could be produced by conventional cell-culture methods. There is evidence that biological products can leak into the blood supply of the animal that is producing them.