**5. Single nucleotide polymorphism (SNP)**

Polymorphism is defined as the presence of two or more clearly different phenotypic variants of a particular DNA sequence in the same population of a species. The most common form of polymorphism is the single nucleotide polymorphism in which variation occurs at a single base pair usually present in approximately 1% of the population. These types of changes can be present in non-coding region of genes and in introns, which would not affect the translation of proteins, but these changes can affect the degree of gene expression and levels of proteins. The changes can also be present in coding regions of DNA or exons resulting in the formation of an altered protein sequence. Sometimes variation in exons do not cause the change in the structure of protein called synonymous polymorphisms.

These changes often produce or eliminate restriction sites for endonuclease to digest the DNA. As a result, fragments of DNA with a different length will be obtained which can be identified by gel electrophoresis. This process is called restriction fragment length polymorphisms (RFLPs). The produced fragments will be the undigested fragments, which is homozygous dominant, whereas the digested fragments are heterozygous and homozygous recessive.

Sometimes polymorphic alleles are linked with each other and within a population in nonrandom proportion is known as linkage disequilibrium (LD), [105] and the combination of alleles (blocks) or set of SNPs present on the same chromosome which tends to be inherited together is termed as haplotype. The size of these blocks is different ranging between 10 and 20 kb and could be important in determining the reason of genetic disorder.
