Jackson has two chromosomal abnormalities. These are mutations in his DNA which mean he cannot produce the proteins needed for normal brain function.
One mutation is called Phelan-McDermid Syndrome, which is a deletion on the 22nd chromosome. The second is some extra 1st chromosome material which is sitting on the end of the 22nd chromosome. These were identified by fluorescence in situ hybridization (FISH) analysis in December 2007. Before this, his family had always thought that he had an extra terminal band on the long arm of the 22nd chromosome as this was as exact as early tests could be.
While Jackson shares many characteristics with others with Phelan-McDermid Syndrome, the addition of the chromosome 1 material make Jackson's disability unique. Jackson is also one of the oldest people in the world identified to have Phelan-McDermid syndrome, as the FISH test has only been recently developed.
How does Jack's disability manifest?
Jackson has no speech, but communicates many of his needs clearly. He is extremely mobile and needs to be supervised at all times.