There's an article in the Australian Journal of Grape and Wine Research that claims to have discovered the genetic parents of the Merlot grape. (The full text of the article is available to subscribers only.)
Scientists had already known that Cabernet Sauvignon and Merlot were siblings, having a common ancestor in Cabernet Franc (the name means "true Cabernet"). Cabernet Sauvignon is (not entirely surprisingly) a cross between Cabernet Franc and Sauvignon Blanc, but the other parent of Merlot was unknown until now.
The researchers have discovered that it's a previously unclassified grape that they have named "Magdeleine Noire des Charentes". It was first discovered growing on the slope of a hill in Brittany; more recently four separate domestic plantings have been discovered in villages around Poitou-Charentes, hence the name.
The researchers also identified the parents of Carmenère (Bastardo, or Cabernet Gros and Cabernet Franc), Merlot Blanc (Merlot and Folle Blanche, a grape mainly used to make Armagnac), Malbec or Côt (Magdeleine Noire des Charentes and Prunelard, a near-extinct grape grown near Tolouse in south-west France, long believed to be related to Malbec and known locally as the Red-Stemmed Côt) and Mourtès - whatever that is, I can find no references anywhere (Magdeleine Noire des Charentes and Penouille, an unremarkable black grape from Bordeaux).