Genetics Index	Glossary

	Chromosomal Translocations

Translocations are chromosomal abnormalities which occur when chromosomes break and the fragments rejoin to other chromosomes. There are many structurally different types of translocations, some of which are discussed below. As with inversions, there is no loss of genetic material, although the breakpoint can cause disruption of a critical gene or juxtapose pieces of two genes to create a fusion gene that induces cancer. In general however, the problem with translocations occurs during meiosis and is manifest as reductions in fertility. Reciprocal translocations In a reciprocal translocation, two non-homologous chromosomes break and exchange fragments. Individuals carrying such abnormalities still have a balanced complement of chromosomes and generally have a normal phenotype, but with varying degrees of subnormal fertility. The subfertility is caused by problems in chromosome pairing and segregation during meiosis. Instead of having homologous chromosomes pair as bivalents, the translocation chromosomes and their homologs must form quadrivalents. Segregation in such a strange situation leads to formation of a number of genetically unbalanced gametes and hence, offspring with unbalanced genomes that are often lethal. This is apparent when one thinks about the types of sperm that could be produced in a hypothetical male with a reciprocal translocation between chromosome 1 and 2, as illustrated below. Note that this animal is a translocation heterozygote, having a normal chromosome 1 and 2 in addition to the translocation chromsomes. The diagram above has been simplified considerably by ignoring the crossovers that occurred during meiosis. In a real testis or ovary, the fraction of gametes that are unbalanced would depend upon how crossingover and segregation occured, which is influenced by factors such as the site of the translocated segments. The net result is considerable difficulty in predicting fertility of a translocation carrier. In general however, they show a substantial (often greater than 50%) reduction in fertility. Some of the offspring of translocation carriers are cytogenetically normal, while others carry the translocation of their parent. Translocations are thus heritable and can be perpetuated in populations. Centric Fusions A centric fusion is a translocation in which the centromeres of two acrocentric chromosomes fuse to generate one large metacentric chromosome. They are also often called Robertsonian translocations, although that term is used by purists to designate a very similar but distinct translocation in which one of the two centromeres is lost. The karyotype of an individual carrying a centric fusion has one less than the normal diploid number of chromosomes. Meiosis in animals carrying a centric fusion chromosome involves formation of trivalents, which is certainly an abnormal structure. Considerable effort has gone into characterizing the effect of this type of translocation on fertility, particularly in cattle and sheep. In general, centric fusions appear to cause a mild reduction in fertility (5-15%), much less severe than in the case of reciprocal translocations. One of the best studied centric fusions is the 1/29 translocation in cattle. This abnormality is quite prevalent in certain breeds, particularly the Swedish Red and White, in which serious efforts have been made to eradicate it.

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Last updated on October 12, 1998

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