ABSTRACT
The ability to isolate, clone and transfer individual genes into farm animals provides the opportunity for scientists to modify the traits of farm animals in ways that were previously impossible. Productivity traits that may be amenable to genetic manipulation include rate of growth, efficiency of feed utilization, nutrient composition of meat and milk products, quality and quantity of wool, and resistance or susceptibility to diseases. Transfer of growth-related genes received the most attention in the beginning of the transgenic era, but progress has been slow and appears dependent on development of effective methods to tightly regulate expression of the transgenes. While alteration of milk composition has tremendous potential for producing entirely new milk products or improving the efficiency of manufacturing cheese, the high cost of producing transgenic cattle and the long generation interval have severely limited this effort. Considerable progress has recently been achieved on introduction of a cysteine biosynthetic pathway into the rumen, modification of wool keratin proteins, and stimulation of wool follicle growth in sheep. If initial success in stimulation of wool growth is substantiated as the transgenic sheep mature, wool may become the first animal product resulting from improvement of a production trait by transgenesis. Some progress has been made on transfer of genes for naturally occurring disease resistance, for preformed antibodies, and for viral envelope proteins. While several strategies to increase disease resistance have potential, additional research is required before these approaches can be applied in the field.
