1. Amaurotic family idiocy, like the juvenile from of Tay-Sachs disease analysed by Sjögren, is determined by a single recessive gene substitution.
2. On this view the expected proportion of amaurotics in fraternities to which they belong calculated from Mendelian theory differs from the observed number in recorded clinical data by an amount less than the standard error of the former.
3. The incidence of consanguineous parentage among amaurotic family idiots is high and accords well with the requirements of the Lenz-Dahlberg formula.