A tandem duplication creates an additional copy of a genomic segment adjacent to the original. A duplicated gene can initially increase dosage, after which the copies can diverge or one can lose function. Adjacency distinguishes tandem copies from duplicates moved to distant positions. Repeated near-identical copies can also complicate genome assembly because short sequencing reads may not identify which copy they came from. [ref]
Talking Glossary of Genomic and Genetic Terms by NHGRI. Duplication entry
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