translocations

Leeds Fetal Medicine Unit Website

Translocations

Chromosomal structural abnormalities can result from breakages within the chromosomes and the chromosomes joining in a different arrangement. If the amount of genetic material remains the same it is called a balanced translocation. If it results in the gaining or loss of genetic material it is known as unbalanced. The majority of balanced rearrangements do not cause any adverse sequelae to the resulting zygote. Unbalanced translocations invariably cause adverse clinical effects which can be severe. New apparently balanced translocations carry a small risk, in the order of 5%, of causing problems due to the possible involvement of the breakage point at a critical region on the gene. The finding of a syndrome in association with an apparent balanced translocation can enable the geneticists to identify the region on a specific chromosome to look at to find the gene responsible. If an apparently balanced translocation is found it is important to check the parental chromosomes to see if it has been inherited from the parent or arisen de novo which will have implications for other family members as well as subsequent pregnancies for the couple.

Robertsonian translocations

Typical Robertsonian Translocation

All chromosomes have a long and a short arm. In the acrocentric chromosomes there is little valuable information contained on the short arm and as such this can be lost without causing any phenotypical effect. If two acrocentic chromosomes each lose their short arms they can become joined together. These are term Robertsonian translocations. The net result is that the parent may only have 45 chromosomes but still have the appropriate amount of genetic material as two chromosomes are joined together. The significance of this is when they come to reproduce at which time they can either pass on the single “double” chromosome, the normal pair both of which lead to normal offspring or they can pass on the “double” chromosome with one of the normal pair which results in a trisomic offspring although if you count the chromosomes there would still only be 46 in number. (See image below). The most common Robertsonian translocations are between chromosomes 13 and 14, 13 and 22, and 21 and 22. The significance of Robertsonian translocations involving chromosome 21 is the increased risk of the children of the affected individual having trisomy 21 which could be up to 100% in the case of a parent carrying a 21/21 Robertsonian translocation. Translocations account for 2-3% of all Down’s syndrome. It is therefore important to test the chromosomes of any infant born with Down’s syndrome to identify possible translocations due to the implications for future pregnancies for the couple.

If parents are known to carry a translocation this will carry an increased risk of miscarriage and a risk in the majority of cases of delivering a living child with anomalies. The risk of each of these two possibilities depends on the chromosome involved and the amount of extra or missing material that might occur. As a general rule the greater the amount of missing or extra material involved the more likely that an unbalanced translocation would result in a miscarriage, whereas the smaller the amount the more likely it would be associated with a viable fetus.

Patient information leaflet on translocations http://www.bshg.org.uk/leaflets_forum/leaflets/Translocation.pdf

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