Harry t21 is a UK family's blog about life with their little boy who has Down syndrome. A recent post draws attention to speculation that Charles Darwin (of Theory of Evolution fame) and his wife Emma's tenth and last child possibly had Down syndrome. Charles Waring Darwin was born in 1857, before John Langon Down's paper identifying the common features of the syndrome was published (1866), and just over 100 years before trisomy 21 was identified as the underlying chromosomal cause (1959). He sadly died at 19 months, from scarlet fever, so it was not explicitly identified at the time.
Randal Keynes's recently-republished biography of Charles Darwin, Creation - the true story of Charles Darwin (2009, John Murray, originally published 2001) discusses the possibility briefly, and includes a short passage, sensitively written by Darwin, indicating that he had noticed some features consistent with what we now know about Down syndrome, and a grainy photograph of Emma with the baby Charles, taken by an older son. The photo is included in the Harry t21 post.
An interesting historical note.