A chromosomal rearrangement in which a segment of a chromosome has inverted from end to end, and re-inserted into the chromosome at the same breakage site. Balanced inversions (in which no net loss or gain of genetic material occurs) are usually not associated with phenotypic abnormalities, however, in some cases, gene disruptions at the breakpoints can cause adverse phenotypic effects, including some known genetic diseases. Unbalanced inversions (in which loss or gain of chromosome material occurs) nearly always yield an abnormal phenotype.