trait, expressed in the phenotype if present in the genotype, which precludes having descendants. All such cases are necessarily sporadic and must represent new mutations as the usual methods of classic genetics provide no means of demonstrating any genetic component whatsoever, except for tenuous arguments such as advanced paternal age. Molecular biology may help although the methods may be tedious; if there is an epistatic gene that may mask the trait, the logic is more tractable, although complex.