| John Langdon Haydon Down (1828-1896) in 1866 first clinically identified the phenotype in patients, that later became known as "Down Syndrome". It was not until 1959, with the development of genetic analysis, that the additional copy of chromosome 21 (Trisomy 21) was identified as the basis of this developmental abnormality. Note that the original paper uses historic terminology that would no longer be considered acceptable in Medicine.
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