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The answer to the question stated above is Duplication. 
Duplication is one of the chromosomal mutation whereby  a segment of chromosome is copied twice, resulting in extra allele. 

Chromosomal mutation  involves the change in chromosome structure which  changes the composition or location allele. 
Other chromosomal mutation includes: 

Deletion - occurs when a segment of chromosome is broken off and lost. 
Inversion - happens when a segment of chromosome is broken off and reattach itself in inverted position. 
Translocation - takes place when a segment of chromosome is broken off and reattach at different point. 

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