I apologise in advance as my response will necessarily feature discussion of racism and ableism

John Langdon Down was a British doctor in the early 19th century, who devoted much of his life to the study and treatment of people with learning disabilities, who were at the time called "idiots" (which is why some people consider this term as offensive as other, more well-known slurs).

Dr Down is credited with identifying what we now call Down's syndrome, but at the time he labelled "Mongolian Idiocy". You see at this time, Charles Darwin's Origin of species was only 5 years old, and although most people accepted that animals could inherit the characteristics of their parents, the mechanisms and degree to which this could affect humans was a hotly debated topic. Many were trying to reconcile these ideas with their spiritual beliefs, as well as what they believed about the "races of man".

The two were often linked, and some contemporary explanations for humans with different appearances included nonwhites being descendants of Cain or other exiled Israelites, or that their appearances were a punishment for having never embraced Christianity, or that they were never truly human to begin with, and so not created in God's image.

The reason this is important, is that Down asserted, in his "Observations on the Ethnic Classification of Idiots" that most learning disabilities represented cases where the superior white race had become degenerate or diseased and so took on characteristics of one of the lesser races, and moreover that this was evidence in favour of humans being the same species, and the races being varieties (although with a strict hierarchy between them) rather than having been created separately.

If these great racial divisions are fixed and definite, how comes it that disease is able to break down the barrier, and to simulate so closely the features of the members of another division.

 

The specific reasons that Down attributed Downs' syndrome to the "mongoloid" race are best left to his own words:

A very large number of congenital idiots are typical Mongols. So marked is this, that when placed side by side, it is difficult to believe that the specimens compared are not children of the same parents. The number of idiots who arrange themselves around the Mongolian type is so great, and they present such a close resemblance to one another in mental power, that I shall describe an idiot member of this racial division, selected from the large number that have fallen under my observation.

The hair is not black, as in the real Mongol, but of a brownish colour, straight and scanty. The face is flat and broad, and destitute of prominence. The cheeks are roundish, and extended laterally. The eyes are obliquely placed, [...] The forehead is wrinkled transversely [...] The lips are large and thick with transverse fissures. The tongue is long, thick, and is much roughened. The nose is small. The skin has a slight dirty yellowish tinge, and is deficient in elasticity [...]

It is worth mentioning, that although these ideas seem incredibly racist to us today (and they still were back then, even if people were unconcerned with it), John Down appears to have spent his life working to help those in his care, and describes in the same paper the physical and mental therapies he has found useful in helping Down's individuals to communicate and develop their motor skills, as well as praising their sense of humour and friendliness - in an age where it was common for disabled people to be treated extremely inhumanely

I'm a geneticist, rather than a real historian, so my main reference is Downs' paper, linked above, as well as the introductory comments added when the paper was reprinted in 1995 as a classic for the journal now called Intellectual and Developmental disabilities.