This study focuses on everyday or lay understandings of inheritance. In the northeastern Brazil, 100 individuals were interviewed in order to describe how they explain the origin of genetic disorders affecting their relatives for several generations. There were involved 60 individuals from a large consanguineous family with many members affected with a neurodegenerative disorder, SPOAN syndrome (spastic paraplegia, optic atrophy and neuropathy), and 40 individuals of another family living with neurofibromatosis type 1 (NF1). The results indicate that families here studied have built narratives to explain the origin of genetic diseases, saying that an ancestor infected with syphilis gave rise to disorders and birthmarks transmitted to descendents.