Genome structural variation

Variation in the organization or copy number of genomic segments, including inversions, insertions, deletions and duplications.

Copy number variation is variation in the number of copies of a particular DNA segment among genomes. A deletion reduces the count; a duplication increases it. For an segment on a non-sex chromosome normally present once on each of two homologous chromosomes, a diploid cell usually has two copies, whereas one additional tandem copy produces a total of three.

The number must be interpreted against a specified baseline. In an ideal comparison with equal sample normalization, three copies versus two give a DNA abundance ratio of $3/2$, or ${\log }_2(3/2)\approx 0.585$ on a logarithmic readout. This arithmetic does not imply that gene expression must rise by the same proportion.

Genome-wide DNA hybridization experiments in 2004 found substantial large-scale copy number variation among otherwise healthy individuals. Such variation is part of ordinary human genetic diversity as well as a possible contributor to disease.^[ref]

Variation among genomes in whether a specified gene or genomic segment is present.

A chromosomal inversion reverses the orientation of a chromosome segment relative to its flanking sequence. Breakage and rejoining can change gene regulation at the boundaries even when the segment retains the same genes. In an individual carrying different orientations, crossing over within the inverted region can produce unbalanced products, reducing recovery of recombinant descendants. An inversion therefore affects inheritance as well as the visual order of chromosomal markers. ^[ref]