The rare condition Williams Beurens Syndrome was listed as of interest to scientists studying domestication because the people born with it are especially gregarious. The syndrome is described as a combination of about 26 missing genes.
Further reading after quick Internet searches revealed that people with this syndrome are exceptionally kind, highly verbally skilled and they have a tendency to be famously musical and possess perfect pitch. The syndrome is characterized by a lack of elastin in the body which gives a certain look to the features.
The Wikipedia entry suggests that these people were thought of as magical because they were so kind and trusting , friendly and musical. They might have been one source of fairy legends.
Origins of fairies