Isochromosome formation is a relatively frequent chromosomal aberration, mainly in X chromosomes. Here the chromosomes are not divided along their length (see the normal division of the chromosomes figure) but transversely.
The resulting isochromosomes ( ) either have two short or two long arms. Persons with this X chromosome anomaly have the same phenotype as patients suffering from (45, X0). This is explained by the fact that a X chromosome arm is missing.
In a reciprocal translocation two broken off chromosome pieces of non-homologous chromosomes are exchanged. This is a relatively frequent anomaly. One finds it with an incidence of 1:500 newborns.
Reciprocal translocations are frequently balanced because the entire genetic material is present. Problems occur, though, in gamete formation.
The fact that translocations occur repeatedly between the two same chromosome segments on various chromosomes says that DNA sequence homologies exist between such chromosome regions that make a healing process of such pieces easier.
Today it is known that balanced translocations can also lead to pathogenic disorders in that proto-oncogenes, which as normal genes in their customary environment are frequently responsible for the controlling cell proliferation, can be transformed into oncogenes through translocation events (connections with other genes).
They are the cause for the origin of many tumors and types of cancer because in other environments they achieve totally different effects. An example for this is the reciprocal translocation between the long arms of chromosomes 9 and 22. Through this, out of a proto-oncogene, arises an oncogene that is responsible for the uncontrolled proliferation of leukocytes in chronic myeloid leukemia (CML).
Another frequently observed anomaly (1:1'000 newborns) is the robertsonian translocation, which occurs between two acrocentric chromosomes of groups G and D. It is also referred to as the centric fusion of two acrocentric chromosomes.
It is a special kind of translocation in that on the acrocentric chromosomes (most often chromosomes 14 and 21 or 22) the very short, satellite-bearing arm is lost and a centric fusion t(14q21q or 14q22q) of the two remainder chromosomes, i.e., the long arms of the two pieces, results.
Carriers of such robertsonian translocations are phenotypically inconspicuous. Also here, though, problems arise when it comes to gamete formation because, normally, the diploid chromosome set is halved thereby. Since, however, in this translocation a chromosome has fused to another one, no ordered segregation can take place. The carrier has a larger probability of having offspring with trisomy/monosomy and this is independent of his age. Often a translocation (e.g., t[14q21q]) is found in families with inherited trisomy 21.