The first diagnosis of a chromosome 18 condition was made in 1963 when a scientist examining chromosome 18 under a microscope saw that a piece of the short arm was missing. This led to the first published report of a person with 18p-. Little did that scientist know that they had barely scratched the surface when it comes to the complexity of chromosome 18 deletions! In the decades since that report, technology has advanced dramatically. We can now examine deletions of chromosome 18 in much greater detail, and some of what we've learned is surprising.

Research completed at the Chromosome 18 Clinical Research Center has shown that some individuals with terminal deletions (involving the tip of the chromosome) have a duplication next to the breakpoint of the deletion. This has been reported with both 18p and 18q deletions. Eight percent of individuals with distal 18q- and 2% of those with 18p- had a duplicated piece of chromosome. Interestingly, these duplications are inverted. In other words, the piece of the chromosome that has been copied has also been flipped. If you think of the chromosome as the alphabet, it is as though the long arm now reads, "JKLMNOPPONM". If you look closely, you'll notice that the letters Q to Z, which represent the tip of the chromosome, are completely missing. The letters PONM are duplicated and reversed. The actual duplications vary in size, from too small to contain a gene to almost an entire chromosome arm.

These types of duplications are not unique to chromosome 18. Terminal deletions with accompanying duplications have been reported on all of the other chromosomes. In fact, the Research Center collaborated with several other institutions to propose a possible molecular mechanism by which these duplications arise.

Of course, the next question is, "What are the effects of these duplications?" Unfortunately, we know less about the effects of duplications of the genes on chromosome 18 than we know about deletions. Therefore, the implication of these duplications is mostly unknown. We suspect that they may contribute at least in part to the variability that we see among individuals with chromosome 18 deletions. As research progresses, we expect to learn more about what these duplications mean for health and development.