Translocation Definition

A chromosomal disorder in which a fragment of a chromosome breaks from its original chromosome and attaches itself to a different chromosome. The fragment may exchange with another fragment, may add itself to another chromosome, or may become lost. Some translocations are random and others occur in predictable patterns. Translocations can be reciprocal, in which chromosome fragments trade places with one another. Such balanced translocations are common and usually do not produce symptoms because all the normal genetic material remains within the genome.

A Robertsonian translocation occurs when the long arms of two acrocentric chromosomes, in which the centromere (waistlike indentation) is so high on the chromosome that the upper arms appear nonexistent and the upper arms contain almost no genetic material. Robertsonian translocations occur only among the five acrocentric chromosomes, which are chromosomes 13, 14, 15, 21 and 22. Like reciprocal translocations, Robertsonian translocations generally do not produce harmful consequences because the genetic material remains unadulterated despite the translocation. Robertsonian translocations are fairly common.

One reciprocal translocation that tends to produce harmful health effects is the Philadelphia chromosome, in which a segment of chromosome 9 and a segment of chromosome 22 exchange places. Geneticists commonly find this translocation in people who have chronic myeloid leukemia (CML).


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