Gene Variations and Single Nucleotide Polymorphism

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Gene Variations

Gene Variations are essentially a change in order of the bases in a DNA sequence. Variations can involve only one or many bases. These variations may be of three different types 1) Polymorphisms- change in one or more nucleotides 2) Deletions- removal of one or more nucleotide and, 3) Insertion- addition of one or more nucleotide. In addition to these changes, a section of DNA on a chromosome may swap places with a part of DNA on another chromosome known as translocation.

The effect of these variations depends upon the region where these variations have occurred. Generally, a majority of gene variations occur in the non-coding region of DNA and thus exhibit no effect at all. Additionally, some variations occur in coding region of DNA and are silent in nature as they do not show any effect. Few variations in coding region do show some characteristic but harmless effect such as difference in heights, color of the iris, shape of the earlobe etc. But, there exist a group of variations in coding and regulatory regions of a gene that has a detrimental effect. These deleterious variations are better known as mutations. These mutations in turn are the reason for many deadly genetic diseases such as Hemophilia, Cancer, Huntington's disease, Cystic fibrosis, Tay Sachs disease, Sickle Cell anemia to name a few. These mutations may be on autosomes {autosome dominant (Huntington's disease) or recessive (Cystic fibrosis)} or on sex chromosomes (X-chromosome linked Hemophilia). Also, some gene variations in coding and regulatory region remain dormant. These variations exert their effect only under certain specific conditions. These changes determine a higher susceptibility of some people to a particular disease and variable response of a drug to different individuals. These dormant variations that do not exert any effect on its own in normal condition but exhibit its effect only under certain environmental/ external conditions are called polymorphisms.

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Genetic Variations may occur in any part of the genetic machinery like gene, DNA, chromosome, protein or protein function.
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