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The chromosome 18 conditions might be rare or even ultra-rare, but chromosome abnormalities in total are common. Since they get named by chromosome region, there are a multitude of names like 1p36, 11q- and dup 22q11.2. The variety of names of conditions obscure the fact they have a unity and a commonness of mechanism; an abnormal number of copies of perfectly normal genes. Conditions caused by the deletion or duplication of regions with multiple contiguous genes make up about 29% of those with intellectual disability.
How many people is that? Let’s delve into the numbers. The official 2025 census number of people in the United States is 341,784,857. Within that population, about 1.2% of the people have intellectual disability. That works out to 4,101,418 people in the US who have intellectual disability. If 29% of those have a chromosomal disorder, then that comes to 1,189,411 people. Over a million people or about 0.35% of the 340 million in the US!
What portion of that million plus people have a chromosome 18 condition? Estimating the number of people in the US with each of these conditions is based on the fact that there are no data suggesting they have a shortened lifespan. The prevalence of 18q- is 1in 54,764 people, therefore there are about 6,241 people in the US with 18q-. The prevalence of 18p- is 1/56,336 therefore there are about 6,067 people in the US with 18p-. The prevalence of Ring 18 is 1/300,129 people, therefore there should be about 1,139 people in the US with Ring 18. And the rarest is Tetrasomy 18p with a prevalence of 1in 625,000 people, so there should be about 547 people in the US with Tetrasomy 18p. In total, that is 13,994 people in US with one of these 4 conditions.
Almost fourteen thousand people sounds like a huge number. But that is still only 1.2% of all of the people with a chromosome disorder. Conditions caused by an abnormal number of copies of perfectly normal genes are collectively common enough that it is surprising there is not a major initiative into designing therapies to treat these types of conditions. In 2004 we tried to create such an initiative by organizing the World Congress on Chromosome Abnormalities. But even after an extremely successful conference, we were not able to get federal funding to support the work of the collective moving forward. Nevertheless, we remain committed to the goal of finding effective treatments. If almost one third of the people with intellectual disability had treatments to help them live healthy and more independent lives, wouldn’t that be amazing!
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