The genetic composition of all living creatures is continually undergoing alteration by mutation, natural selection, and genetic drift. Beginning at the onset of plant and animal domestication, humans have further manipulated the genetic composition of plant and animals to enhance their health and usefulness to humans by selecting for specific phenotypic traits. Development of recombinant DNA technology has enabled scientists to isolate single genes, analyze and modify their nucleotide structures, make copies of these isolated genes, and insert copies of these genes into the genome of plants or animals. The procedure used to insert these isolated genes is called gene transfer; an animal that contains the inserted gene or genes is called a transgenic animal; and the transferred gene is called a transgene.
The first intentional transfer of a transgene into the genome of an animal was achieved in 1980 in mice. Gene transfer methodology was subsequently successfully applied to pigs, sheep, and rabbits. Gene transfer has thus far been used most extensively for basic research on all aspects of biology and genetics, but it has numerous potential applications for genetic improvement of farm animals. Practical applications of gene transfer in livestock production include improved milk production and composition, increased growth rate, improved feed use and carcass composition, increased disease resistance, enhanced reproductive performance, and increased prolificacy. Gene transfer in farm animals has also been investigated extensively for potential to produce human pharmaceutical products, and the alteration of cell or tissue characteristics has been investigated for potential use in organ transplantation in humans.
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