**7. Conclusion**

Polymorphism is a variation in DNA sequence that may affect individual phenotypes. It occurs more often in the general population than mutations (frequency ≥ 1%). The majority of variation is single nucleotide polymorphism (SNP) which is a single base change in a DNA sequence that occurs at a specific position in the genome. SNP may locate within coding, or non-coding, or intergenic regions of genes. Most of SNPs have two alleles, for an individual SNP, one is major allele and the other is the minor allele based on their observed frequency in the general population. Genome-wide association studies (GWAS) search for SNPs that occur more frequently in person with a particular disease than in person without the disease and pinpoint genes that may contribute to risk of disease. Linkage disequilibrium (LD) is commonly used to indicate that two or more genes are physically linked. It plays an important role in health and disease. Most SNPs are not responsible for a disease state but serve as biological markers for various complex diseases such as cancer, diabetes, dyslipidemia, schizophrenia, and so on. There are several methods for analyzing SNPs such as MALDI-TOF MS, GeneChip® array, pyrosequencing, DHPLC, HRM, RFLP, and so on.
