Methods of gene transfer
The technology employed for introduction of transgenes into a livestock population may produce consequences specific to the method. Once introduced, transgenic animals will become subject to the traditional selection criteria of livestock producers and the market forces for the gene products involved. A transgene will likely be expanded into the breeding population so long as the associated trait remains desirable in production systems and the marketplace. By use of marker-assisted selection, a few transgenes could be selectively incorporated into or eliminated from an otherwise diverse breeding population without great difficulty. The method resulting in Polly is seen as a major improvement as compared to the technology of the early nineties that led to the first transgenic bovine creature, carrying the human lactoferrin gene (lactoferrin is an important component in breast milk). At that time, genes were injected into newly fertilized eggs, and only in rather infrequent cases did the gene stably integrate and lead to a transformed animal.
Although transgenic mice were produced three decade ago, the mouse is still the animal of choice and used as model system for most of the experiment. However, currently pigs are used for most experiments because of its potential transgenic frequency. Following are some of the gene transfer methods carried out in animals.