Thalidomide may be a mutagen

Mutagens attack genes at random. [...]mutagenesis might equally well result in achondroplasia or neurofibromatosis or any other genetic condition in which new mutations are frequent. The grandparents are reported as unaffected, which suggests that a new mutation has occurred at some point in each pedigree, as frequently happens with these dominant malformation syndromes. Since each father was exposed to thalidomide in utero it is quite possible that the fathers' malformations were caused by thalidomide - or maybe by a combination of a genetic predisposition and the teratogen.