Abstract
Down syndrome (DS) and Marfan syndrome (MFS) are two unique genetic disorders that share limited phenotypic overlap. There are very few reported cases in the existing literature on overlapping DS and MFS. Although these two disorders are phenotypically unique, features present in these cases are variable, resulting in mixed and dominant expressions of particular features. We present the first adolescent case of trisomy 21 associated DS and fibrillin-1 gene associated MFS in the literature who had a height at 90th percentile for an 11-year old boy and discuss the implications of this case in terms of future medical care when these two genetic syndromes are present in the same individual. Understanding of certain features of the 'non-dominating' syndrome is crucial for clinicians to recognise when DS co-occurs with MFS. Close monitoring of the cardiovascular, ophthalmologic and musculoskeletal systems is recommended if both syndromes are diagnosed given that both can be independently associated with disorders in these organ systems.