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In contrast to deletions, duplication mutations occur when a segment of DNA is copied one or more times. These mutations often happen because of errors during genetic recombination (see Figure 4) (Strachan & Read, 2018).
Small duplications cause certain DNA sequences to appear repeatedly, leading to changes in copy number variation (CNV). This means that some genes may be present in more or fewer copies than normal, which can affect how much of a protein is produced.

Changes in repeat numbers can cause genetic disorders. A well-known example is Huntington’s disease. The huntingtin gene (HTT) normally contains 27–35 repeats of the three-nucleotide sequence CAG. When the number of repeats exceeds 40, it leads to a progressive neurodegenerative disorder affecting the central nervous system (NHGRI, 2023).

Large duplications involving whole genes or chromosome segments can increase gene expression and are often associated with diseases such as cancer, where excessive cell growth occurs.