Transgenic or genetically modified organisms are those species which have foreign genes in their genetic makeup to show certain characteristics. Recombinant DNA technology is used for this purpose.

When there were not technologies like biotechnology or genetic engineering, the farmers would breed their animals selectively for the increased production of milk. It was in 1970 that the techniques of genetic engineering were applied on mice and first mouse was produced by combining the cells of two different embryos of different strains. a single embryo was developed in this way which when developed into a complete organism, it showed the characteristics of both the strains.

Then gradually genetic engineering started making progress and developed more techniques like DNA microinjection to develop transgenic animals. This technique was also applied on mice for the first time. Then many other animals were produced who had genetic modifications in their genomes.

How transgenic Animals are developed:-
Mouse is chosen for performing the genetic engineering experiments due to its small size and it reproduces in a very short time. There are three methods which are used to convert the normal animals into transgenic animals.

Embryonic stem cell-mediated gene transfer:-
In this technique, the foreign DNA is inserted into the embryonic stem cells culture in in vitro environment by homologous recombination. Stem cells are the cells other than the sex cells which have the ability to differentiate into different types of cells giving rise to a complete organism. If the study of the genetic control is concerned then this technique is of significance and it is best used mice. It is also able to target the mutations in the genes by homologous recombination.

DNA Microinjection:-
DNA microinjection is concerned with the insertion of a single gene or group of gene into the pronucleus of a fertilized ovum. The genes of interest are taken either from the same species or from different species. This method is best used in mammals where it introduces new DNA. The introduction of new DNA may lead to the expression of either certain genes within the mammal or the expression of those genes which are newly added into the animal's genome. There is also this possibility that the gene inserted may not reach the desired site in the genome. The genetically modified ovum is then transferred into the recipient female. This method can be applied in various species.

Retrovirus-mediated gene transfer:-
Sometimes the new genes are added into the genome of the animals to increase the expression. To insert new genes, retroviruses are used as vectors which are responsible for taking the genetic material into the organism's genome. The genes inserted through this method are not expressed in all the cells. If the retrovirus succeeds to enter the germ cells of the organism, it is possible that the transgenes will be expressed in al the cells in the next generation.

Application of Transgenic animals:-

In biotechnology, the transgenic animals can be used in the research purposes. Using the techniques of recombinant DNA technology or genetic engineering, the health of animals can be enhanced by making changes in their genes. Production of milk and meat also increases by using the growth hormones in these animals.

Medicine is one field which makes use of transgenic animals most. If the researchers have to see the expression of certain genes, they insert those genes in animals and make them genetically modified. Similarly if the diseases of humans are treated, then those diseases are first developed in the animals by making mutations and then the effects of the disease are observed and certain drugs are developed after that.

For the purpose of drug delivery or drug discovery, transgenic animals are used. The newly developed drugs are also tested on these animals.

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