Modification of performance traits
Modifications to livestock that could influence traits such as growth rate, feed efficiency and meat quality are major goals for livestock breeding in agriculture. Mice in which the myostatin gene has been knocked out have considerably more skeletal muscle than wild type controls. The double-muscle phenotype of some breeds of cattle is caused by natural mutations of this gene and these are widely bred in Europe for improved meat quality. Knockout of this gene in sheep or pigs could produce animals with increased skeletal muscle mass and improved meat quality, although there are clearly major ethical issues relating to animal welfare that would have to be considered if these types of animal were generated.
Other areas of transgenic applications in animal agriculture include modifying milk composition to increase milk proteins, to decrease milk fat or to change milk composition to provide a source of milk for lactose-intolerant individuals or to provide a source of milk that is more efficiently processed into cheese. An area that definitely needs more exploration is the use of the chicken oviduct as a bioreactor. This approach has numerous advantages over the transgenic mammary gland. To name a few, the ease of collecting the product, short generation times, quantity, safety and low costs are unmatched by the traditional approach of transgenic milk. However, this system is still in its infancy because micromanipulation of the chicken embryo is very difficult, and transgenic chickens are often mosaics. It is also possible that the best protein expression levels will be obtained when multiple copies of transgene integrate into a particular favourable chromosomal locus. Cloning will greatly reduce timeliness to get therapeutic products to the market by producing large number of genetically identical transgenic livestock animals in the first generation.