This article critiques how the chromosomal condition ‘trisomy 21’ (‘T21’) (‘Down syndrome’) was originally conceptualised using colonial, scientific and medical discourses on ‘race’ and ‘idiocy’. Nineteenth century discourses surrounding ‘degeneracy’ commonly intertwined the notions of ‘race’ and ‘idiocy’. In Observations of an Ethnic Classification of Idiots, Down categorises people with T21 as ‘Mongolians’ because of their purported similarities to ethnic ‘Mongolians’. The discourse-historical approach (DHA) to critical discourse analysis (CDA) is used in this article to examine how the ‘Mongolian idiot’ (...) is constructed by Down as the ‘other’, being simultaneously a ‘racial other’, a ‘disabled other’, and an ‘idiotic other’. Down theorises that ethnic ‘Caucasians’ with ‘idiocy’ have ‘retrogressed’ to primitive forms as characterised by the ‘Mongolian family’. His report also essentialises the ‘Mongolian idiot’, defining their ‘degeneracy’ and ‘otherness’ in terms of their physical, behavioural, and mental characteristics. Whilst Down’s ‘mongolism’ has since been refuted, aspects of his report are reproduced in some contemporary medical and scientific discourses. Although these discourses are less essentialist than Down’s, they persist in constructing the ‘otherness’ of T21, by foregrounding its ‘abnormalities’ and ‘disorders’. Significantly, facets of ‘degeneracy’ appear in evolutionary developmental biology, connecting T21 with ‘atavisms’, though without the racial framework. (shrink)