Williams-Beuren syndrome

Williams syndrome (WS or WMS; also Williams–Beuren syndrome or WBS) is a rare neurodevelopmental disorder characterized by a distinctive, "elfin" facial appearance, along with a low nasal bridge, an unusually cheerful demeanor and ease with strangers; developmental delay coupled with strong language skills; and cardiovascular problems, such as supravalvular aortic stenosis and transient hypercalcaemia. It is caused by a deletion of about 26 genes from the long arm of chromosome 7. The syndrome was first identified in 1961 by Dr. J. C. P. Williams of New Zealand and has an estimated prevalence of 1 in 7,500 to 1 in 20,000 births.

Williams-Beuren syndrome in It's the End of the World As We Know It, And We Feel Fine
It was known, back in the day, that individuals with a varient in the WBSCR17 gene developed Williams-Beuren syndrome. They looked "elfy-welfy", the bridges of their noses were abridged, and they were much more friendly, gregarious and trusting.

After the Big Fracas, the survivors engaged in genetic research leading to the development of a WBSCR17 variation that had the positive effects of Williams-Beuren syndrome without the negative (mental retardation, heart disease). All survivors were genetically engineered to have this variation leading to today's more harmonious society without the risk of another Fracas. Occasional throwbacks, while treated kindly and allowed to live freely with minimal constraint, were not allowed to reproduce.