What is a change in the usual order of a dna sequence called

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The answer is Mutation.

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Mutation. There are multiple types of mutations. Substitution mutations are when a nucleotide is switched in the genetic sequence. Insertion is when a nucleotide is inserted into the DNA sequence. Deletion is when a base nucleotide is deleted. Those are the three basic mutations of DNA. Even though they are very small, they can have a profound impact on the organism. In the case of sickle cell anemia, one substitution causes red blood cells to completely change shape from their normal circular shape to a sickle shape. 
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