Chromosome structure abnormalities can be either unbalanced rearrangements or balanced rearrangements. Balanced rearrangements usually do not lead to phenotypic abnormalities; all genetic information is present in the right amount, just in a different order or in the wrong location.
An inversion occurs in two steps: a chromosome breaks in two locations; the chromosome refuses with the segment between the breaks inverted. When both breaks are in the same arm, the inversion is paracentric; when there is one break in each arm, the inversion is pericentric.
Paracentric inversions do not usually affect phenotype, and are hard to detect using banding or FISH. However, offspring of an inversion carrier are at risk because a loop forms when paternal and maternal chromosomes cross over; aberrant recombinations can occur within this inversion loop. Pericentric inversions, though, can lead to unbalanced gametes with duplication and deficiency of chromosome segments.
Translocation is the exchange of chromosome segments, usually between nonhomologous chromosomes. There are two primary types of translocations: reciprocal translocations and Robertsonian translocations. A reciprocal translocation occurs when segments from nonhomologous chromosomes break off, with segment attaching to the other chromosome and vice-versa. A Robertsonian translocation occurs when two acrocentric chromosomes (chromosomes with tiny short arms, meaning the centromere is near one end) fuse at the centromere and lose their short arms.
Reciprocal translocations usually involve only two chromosomes, meaning the total chromosome number is unchanged. Reciprocal translocation are typically harmless, despite being more common in individuals so retarded that they require institutional care. There are three kinds of reciprocal translocation: alternate; adjacent-1; and adjacent-2.
Robertsonian translocations lead to a balanced karyotype with only 45 chromosomes. Because acrocentric short arms contain just multiple copies of ribosomal RNA genes, loss of acrocentric short arms is not phenotypically important.
A third type of translocation -- an insertion -- occurs when a segment from one chromosome inserts into a different chromosome (either in its original orientation, or inverted). Inversion translocations are rare because they require three chromosome breaks. Abnormal segretation in insertion carriers can lead to offspring with deletions or duplications of the inserted segment. Alternatively, offspring can be normal or balanced carriers.