Structural Changes in Chromosome

A structural chromosomal aberration rearranges the genes located on a chromosome, without necessarily changing the overall chromosome count, and arises when a chromosome breaks (typically because of ionising radiation or a chemical mutagen) and the broken pieces rejoin incorrectly. Based on the pattern of breakage and reunion, there are four recognised types, grouped into two categories. The first category changes how many copies of a gene locus are present: a deletion (or deficiency) is the outright loss of a chromosome segment, occurring either as a terminal deletion (a single break near one end removes the segment beyond that break) or as an intercalary/interstitial deletion (two breaks, with the flanking pieces reuniting, remove only the internal segment between them); both types have been observed in Drosophila and maize, are visible during meiotic pachytene and in polytene chromosomes as an unpaired 'deficiency loop' where the normal homolog bulges out because it has no matching sequence to pair with, and a sufficiently large deletion is typically lethal. A duplication (or repeat) is the opposite change, where the same segment of genes appears more than once in a chromosome, first reported by Bridges in 1919 in Drosophila and also documented in maize and pea; it comes in three arrangements -- tandem duplication, where the repeated segment sits immediately after the original copy in the same gene order; reverse tandem duplication, where the repeated segment is adjacent but its gene order is flipped; and displaced duplication, where the repeated segment appears elsewhere on the same chromosome, away from the original. The second category rearranges gene order without changing how many copies exist: an inversion is a 180-degree flip of a chromosome segment, requiring two breaks and a reunion, first reported by Sturtevant in 1926 in Drosophila, and is termed paracentric if the inverted segment lies entirely to one side of the centromere or pericentric if the inverted segment spans the centromere; and a translocation is the transfer of a segment from one chromosome to a completely different, non-homologous chromosome (not to be confused with ordinary crossing over, which exchanges material only between homologous chromosomes), occurring as a simple translocation (a single broken segment attaches to the end of a non-homologous chromosome, very rare in nature), a shift translocation (a broken segment inserts internally, rather than at the end, into a non-homologous chromosome), or a reciprocal translocation (a mutual exchange of segments between two non-homologous chromosomes, sometimes called illegitimate crossing over, and further split into homozygous translocation, where both chromosomes of each of t …