Origin of the deviating chromosome structure
Structural aberrations are the result of chromosomal breaks that occur during meiosis. Here deletion, duplication and isochromosome formation lead to an abnormal phenotype, while insertion, inversion as well as translocation can be balanced. This means that the carrier of this structural chromosome aberration can escape notice phenotypically, because the entire genetic material is present.
A deletion can happen in every chromosome and exhibit every size. The consequences of a deletion depends on the size of the missing segment and which genes are found on it.
A partial deletion on the short arm (p) of chromosome 5, for example, is responsible for the "cri du chat" syndrome.
Ends (telomeres) of chromosomes sometimes break off and are lost. In this case the formation of chromosome rings can take place in that the two ends bind to one another. If no genetic information is lost, such structural changes are balanced and the phenotype is inconspicuous. Problems only arise in gamete formation.
A chromosome duplication is the doubling of a chromosome piece. A duplication is sometimes termed a "partial trisomy". If, therefore, a duplication is present, the person is equipped with 3 copies of the genes in the associated chromosome segment.
This means that extra directions (genes) are present, leading to congenital abnormalities or developmental problems.
If chromosome pieces that have been broken out become inserted again, but reversed, an inversion has occurred.
The phenotype of this disorder is usually unobtrusive, since the entire chromosomal information is still present. When the interchanged region includes the centromere, one refers to it as a pericentric inversion, otherwise to a paracentric inversion.
If chromosome pieces are reinserted somewhere else, this is referred to as an insertion. Carriers of such insertions can be phenotypically inconspicuous because no information has been lost.