Gene duplication is one of the key factors driving genetic innovation,
i.e., producing novel genetic variants. Although the contribution
of whole-genome and segmental duplications to phenotypic
diversity across species is widely appreciated, the phenotypic
spectrum and potential pathogenicity of small-scale duplications in
individual genomes are less well explored. This review discusses
the nature of small-scale duplications and the phenotypes produced
by such duplications. Phenotypic variation and disease phenotypes
induced by duplications are more diverse and widespread
than previously anticipated, and duplications are a major class of
disease-related genomic variation. Pathogenic duplications particularly
involve dosage-sensitive genes with both similar and dissimilar
over- and underexpression phenotypes, and genes encoding proteins
with a propensity to aggregate. Phenotypes related to human-specific
copy number variation in genes regulating environmental responses
and immunity are increasingly recognized. Small genomic duplications
containing defense-related genes also contribute to complex
common phenotypes.
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