Chromosome 18
There are many genetic disorders due to gene mutations on chromosome 18. For example 'tetrasomy 18p' which results in an isochromosome (an extra chromosome) with an extra p or q arm. This causes an intellectual disability and delayed development for the sufferer. Also 'trisomy 18p' is a result of aquiring 3 of chromosome 18 insead of the original 2 copies leaving the suffere with great birth defects which could be potentially fatal.
(http://ghr.nlm.nih.gov/chromosome/18) accessed on 22nd October 2013