Answer
(a) Transgenic animals are so called because they have their DNA manipulated. As a result they possess and express foreign gene.
(b) Role of in vaccine safety
Transgenic mice are being used to test the safety of the polio vaccine before they are used on humans.
Biological products
Medicines required to treat many human diseases contain certain biological products which are expensive to make. These products can be produced by transgenic animals by induction of genes in them which code for particular product e.g. human protein (alpha -1 - antitrypsin)