The Prelude to Dune prequel novels reveal that Mohiam is secretly the mother of Jessica and grandmother of Paul. According to the authors, this fact was pulled directly from Frank Herbert's working notes for the original Dune series.
Mohiam is instructed by the Bene Gesserit to collect the genetic material of Baron Vladimir Harkonnen (through conception) for their breeding program (it had been previously established in Dune that he is secretly Jessica's father). She blackmails the homosexual Baron into fathering a child with her. When that daughter proves genetically undesirable, Mohiam kills it and returns to Harkonnen for a second try; at this point he drugs and viciously rapes her. She exacts her retribution by infecting him with a disease that later causes his obesity. Mohiam's second child with Harkonnen is, of course, Jessica.
It is also revealed in the prequels that the name given for Jessica's mother in the original novels, Tanidia Nerus, is an alias used by Mohiam. This name is presumably the name listed in the "open" records, as the Bene Gesserit commonly hide true parental identity.
Read more about this topic: Gaius Helen Mohiam