Chromosomal translocation

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Several sorts are described, with different significance and effect.

Acquired translocations can be a causative factor in carcinogenesis, e.g. the Philadelphia chromosome, characteristically associated with chronic myeloid leukaemia.

The notation is to write t(A;B)(pxx;qyy) where A is the recipient, B is the area broken off. The second set of parentheses gives more detailed location of the break, where the notation is p for the short arm (petit) and q the long arm. The numbers following the p or q are bands and sub-bands that further pin-point the point of the break. Strictly speaking, these should therefore be read as individual digits, not as whole numbers, e.g. X11.2 refers to band 1, sub-band 1, sub-sub-band 2, and should be read as one-one-two rather than eleven-point-two.

The Philadelphia chromosome is a t(9;22)(q34;q11) translocation, i.e. chromosome 9 receives a broken part of chromosome 22, resulting in a elongated, abnormal chromosome 9 and a shortened chromosome 22.

Balanced Robertsonian translocation

Chromosomes with very short p arms are described as acrocentric (acro means extremity) - these are 13, 14, 15, 21, and 22. If there is abnormal fusion of two acrocentric chromosomes such that the two short arms fuse, the end result is that two centromeres very close together with two long arms on either side. Frequently, one of the centromeres will become inactivated, so that chromosomal segregation can occur normally.

Technically, this is an unbalanced translocation as material on the short arm is lost, but as so little chromosomal material is lost, there is no or minimal effect on the phenotype of the individual.

The individual will be aneuploid as there are only 45 chromosomes. However, although there may be no symptoms in the individual, their children may suffer monosomy or trisomy of the involved part of one of the chromosomes.

Isochromosome

Not strictly a translocation, but occurs when a chromosome breaks and one arms replicates, resulting in the chromosome formed of two p or two q arms. Denoted, for instance, by i(12p). There is loss of genetic material on the other arm and duplication of material on the duplicated arm (unless imprinted, i.e. epigenetically silenced).

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