Down syndrome is the most common genetic cause of learning difficulties and has a
prevalence rate of 1.08 in every 1000 live births (Morris & Alberman, 2009). The most common form of Down syndrome is trisomy 21, which is caused by an extra copy of chromosome 21. Down syndrome can also be caused by mosaicism, when only some of the cells in the body are affected with trisomy 21, and Robertsonian translocation, when a section of chromosome 21 becomes attached to another chromosome during cell division. Down syndrome is typically associated with an IQ of approximately 50 although there is wide variation (Chapman & Hesketh, 2000; Määttä et al., 2011).