**2.1.1 DNA- polymorphism**

Polymorphism literally means many forms and it is seen that on an average, every 1 in 500 base pairs of DNA varies between individuals. Variations in coding regions are rare because of the need to preserve function. Changes in non-coding regions of the genome are subject to very little selection pressure and generate allelic variations at a very high frequency. This variation can be an alteration in a single base, deletion or addition of bases and expansion or contraction of repeats.

DNA polymorphisms are the basis of all current *genetic markers*. A marker is any *polymorphic Mendelian* character that can be used to follow a chromosomal segment through a pedigree. Allelic human gene *expression variation* may be caused by changes in regulatory DNA, including sequences which regulate transcription and splicing. This type of sequence variation may often underlie the susceptibility to common diseases but quantitative methods to explore allelic variation in human gene expression have been developed only very recently (Yan H et al, 2002).
