An infant with a karyotype of 48, XYY, G + presented with the usual findings of Down's syndrome except for the absence of epicanthal folds and a prominent occiput. This condition would be expected to occur at a frequency of 1:480,000 male births. This estimate is based on the assumption that the two events occur independently. Analysis of blood groups, haptoglobins, and dermatoglyphics was unrevealing. No clear modifications of the mongoloid phenotype by the additional Y are apparent.
Al-Aish MS, Dodson WE, Plato CC. Down's Syndrome With XYY: 48,XYY,G +. Am J Dis Child. 1971;121(5):444-446. doi:10.1001/archpedi.1971.02100160114020