Variations in DNA sequences underlie the differences among different members of the same species and also between different species. DNA sequence variations are usually known as polymorphisms if the commonest allele is less than 0.99 in a given population.
DNA polymorphisms are widespread in many different species, particularly in humans. Examples include single nucleotide polymorphisms (SNPs), microsatellites, minisatellites, small insertions/ deletions, and large insertions/deletions. DNA polymorphisms may not haveany phenotypic effect at the protein level or at the level of the whole organism.
On the other hand, they are usually called disease-causing or pathogenic mutations if they cause a change in the phenotype and results in a disease status. The frequencies of individual mutations are usually not high because of selection pressure against such less favorable base changes. It is thus important to study DNA sequence variations in various branches of biological sciences.
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