Ring Chromosome 18

Although it is ultra-rare, ring chromosome 18 (RC18) is one of the most commonly occurring congenital ring chromosomes (RCs). This is likely due to the fact that chromosome 18 has the second smallest number of genes of all the autosomes, thereby limiting the potential genetic ramifications. The small number of genes involved likely also underlies the finding that an early death rarely occurs in affected individuals who do not have the rare phenotypes of holoprosencephaly or congenital heart disease. The morphology of the ring is identified microscopically while the net content of the chromosome requires molecular techniques such as array-based comparative genomic hybridization (aCGH). A consistent clinical description is complicated because no two individuals have the identical genetic constitution. This inter-individual variability is due to two factors. First, there is a high probably of mosaicism due to the mitotic instability of RCs. Second, the content of the RC18 is unique to each individual. This includes different regions of hemizygosity for both the long and short arms of chromosome 18 plus a high probability of duplications within the RC. This structural variability is likely due to the fact the chromosome 18 does not contain repeat sequences that can drive recurrent recombination.