Transgenic animals are genetically engineered animals in which there is addition of exogenous DNA, leaving host DNA intact. The modification of germ cells by gene manipulation in vitro is referred to as transgenesis. The term ‘transgenic’ in case of animals was first given by Gordon and Ruddle. The ‘transgenic’ word is now used to describe particular variants of species into which anew gene has been inserted. Transgenic animals are created by introducing new DNA sequences into germline via addition to the egg. In animal breeding, modification germ cells and improvement of the animals are desired goals. The objective of transgenesis is to produce animals with heritable change in their genotype, so that the benefits of gene manipulation can be passed into their offsprings. Desirable features of transgenic farm animals include increased efficiency of feed utilization, leaner meat, more rapid growth to marketable size and increased disease resistance.

Methods of producing transgenic animal

There are five basic methods now used in the development of transgenic animals by getting new genes into animals. These are as follows:

(1) Microinjection of eggs
(2) Use of stem cells
(3) Virus vectors
(4) Direct uptake of DNA stimulated by calcium or an electric current
(5) Use of liposomes

1. Microinjection of eggs – A new gene must be introduced into an egg cell and by this way all the cells of an animal contain a new gene. This is done firstly by giving a hormone fertility drug to a female to stimulate production of extra eggs by the ovary. Fertilisation is allowed to occur and then the fertilized eggs are collected.

The donor DNA is then injected directly into one of the pronuclei of a fertilised egg using a very fine needle like pipette while viewing under microscope. In some cases, the DNA integrates into one or more of the chromosomes. The two pronuclei later fuse and egg becomes the zygote. The fertilized eggs are then transferred to one or more foster mothers and the offspring are later screened for the presence of new gene. The best success rate achieved so far is about one transgenic animal for every 20 eggs treated (sheep) and 100 eggs (cow). The first experiments on farm animals were carried out on rabbits, pigs and sheep and these have been followed by cattle and fish.

2. Virus vectors – In principle, this method is similar to that used for phage vectors in bacteria. Plasmid vectors are not responsible because animal cells do not contain plasmids. It is not used with egg cells but only when some of the body cells need to be transformed.

3. Liposomes – These are small artificially created spheres surrounded by a phospholipids bilayer like a membrane. The required DNA is contained within the liposome. The liposomes fuse with and enter the cells.

4. Direct uptake of DNA – Fragments of DNA can be taken up directly by phagocytosis under the right condition. It is best suited gene therapy where only some of the cells in the body need to be modified.
Where somatic cells are to be modified the use of viral vectors or DNA uptake methods gives satisfactory result. For the modification of germ cells microinjection method is more convenient.

5. Use of Stem cells – In this method a few cells (known as ‘stem cells’) are taken from a young embryo. These cells can be cloned indefinitely in a test tube. The new gene can be introduced into the cells by various means including microinjection. The advantage is that the cells that are expressing the new gene (transformed cells) can be identified before adding them to the foster mother. This saves producing many unwanted non-transgenic animals.

The successfully transformed cells are injected back into a normal embryo and become part of its normal development. The resulting animal is a mixture of two genetically different types of cell, some derived from the transformed stem cells and some derived from the normal stem cells. Such an animal is known as chimera. The sex cells of the animal will also be mixed and some of its gametes will carry the new gene. These will give rise to completely transgenic animals in the next generation.

Production of Transgenic animals

1. Transgenic Mice – One of the earliest successes in creating transgenic animal was in a mouse. A growth hormone gene from a rat was inserted into the genome of a mouse. Attached to the growth hormone gene was a powerful promoter which was stimulated by the presence of heavy metals in the mouse’s diet. When these heavy metals were included in the mouse’s food, the growth hormone gene was almost continually ‘switched on’. This made the mouse to grow at 2-3 times faster than mice without the gene. The mouse with the growth gene also finished growth at about twice as large as normal. This was achieved through genetic engineering.

2. Transgenic sheep – The human growth hormone genes were introduced into animals like sheep. When this was done the normal controls overproduction of the hormone was avoided. Transgenic sheep which overproduce growth hormone grow leaner and put on weight more quickly making more efficient use of their food. However, they are more prone to infection, tend to die young and the females are infertile.

3. Transgenic Fish – More recently scientists in Canada have added a new gene from another fish to salmon which activates the salmon’s own growth hormone gene. The salmon grow upto 30 times their normal weight and at 10 times the normal rate. Scottish fish farmers started breeding them on a trial basis in 1996.

Examples of Transgenic Animals

Most new scientific and medical procedures are still tried first on mice because they breed so rapidly and are convenient to handle. Some of the important examples of animal transgenesis which had been achieved by the mid 1990s are mentioned here. A list of animals that have been genetically manipulated and some of the characteristics involved are given below:

(a)  For production of pharmaceuticals –
Cattle, Goat, Mice, Pigs, Rats and Sheep
(b) More rapid growth –
Carp, Catfish, Cattle, Pigs, Sheep, Salmon
(c) Alterations for research on diseases –
Mice, Pigs, Rats
(d) Tolerance to low temperature - 
Salmon, Seabass

Usefulness

Transgenic technology has been particularly useful in immunology, oncology, and studies on genetic basis of diseases and in gene therapy. Transgenic animals have been created for their potential use in diverse fields. These animals are disease resistant and provide meat and milk at a lower cost of production. Besides, they serve as bioreactor producing useful drugs, vaccines, hormones, organs for transplantation and many other products benefiting mankind.

Limitations

While transgenic mice are often used as an experimental tool, this technique can not be always applied to farm animals to derive commercial benefits. It is too difficult to be applied to large domestic animals due to following reasons.

1.    They produce not more than two offspring.
2.    Re-implantation of manipulated embryo is more difficult for farm animals such as sheep and cattle.
3.    The eggs of many domestic animals have opaque cytoplasm where their pronuclei or nuclei can not be visible.
4.    The enhanced growth of mice after the transfer of human growth hormone gene is an effort which is not easy to apply for other domestic animals.

However, Genetic engineering is most powerful technique available in applied genetics and biotechnology. It gives us the power to study and to change the genetic instruction of an organism including ourselves. Other living organisms can be changed for the benefit of human beings and we are even beginning to manipulate our own genes to cure genetic diseases.