Creation of Transgenic Farm Animals
Transgenic animals have numerous applications in agriculture, medicine, and industry. The first transgenic mouse was created in 1985, followed a few years later by genetically modified rabbits, pigs, sheep, and cattle. Rationales for the genetic engineering of farm animals include improving the nutritional value of meat, milk, and eggs; making animals resistant to veterinary diseases such as mastitis and mad cow disease; and producing human pharmaceuticals in the milk of transgenic cows, goats, or rabbits.
For example, at the University of Illinois in Urbana-Champaign, Prof. Matt Wheeler took a gene that increases milk production in dairy cattle and injected it into hundreds of pig embryos, only one of which survived with the new gene intact. Several offspring of this animal inherited the cow gene and began to produce more milk, nursing litters that gain weight faster before weaning.11
Method for Gene Transfer in Animals
The most common method for producing transgenic animals is gene transfer by DNA microinjection, which involves the following steps:
DNA containing the desired transgene is identified and cloned (copied tens of thousands of times in bacteria) before insertion into the animal host.
The host animals (cows, pigs, or sheep) are induced to superovulate and their eggs are collected.
The eggs are fertilized in a laboratory dish.
Using a fine, hollow needle, a solution of DNA containing the transgene is injected into the male pronucleus of the fertilized egg (the nucleus of the sperm cell that entered the egg) before it fuses with the female pronucleus.
The transgenic embryos are grown in cell culture and then implanted into the uterus of a surrogate mother, where they complete their development.
Screening is performed to determine which of the offspring have inherited the transgene. The main drawback of DNA microinjection is its low success rate: only between 1 and 4 percent of microinjected eggs result in the live birth of a sheep, goat, or cow containing the transgene, and about 80 to 90 percent of transgenic embryos die during early development.11