The most commonly reproduced pedigrees in the medieval Welsh corpus were undoubtedly those of the kings of Gwynedd. They were included in all the major genealogical collections discussed in the preceding chapters and commonly appear in manuscripts as individual items, owing largely to the prominence of the Gwynedd dynasty in the twelfth and especially thirteenth centuries. Underlying most such pedigrees is the Merfynion pedigree of the ninth and tenth centuries, which traced the Merfynion descent through Merfyn's mother Esyllt to the earlier kings of Gwynedd as far back as Cunedda Wledig, whose own descent was then traced to Beli Mawr. This chapter examines how and why these Gwynedd pedigrees were successively elaborated between the twelfth and fifteenth centuries.

Following a brief discussion of the extant early versions of the Merfynion pedigree traced through Cunedda Wledig, the discussion is divided into five major sections. The first section attempts to explain how Beli Mawr's own ancestry was extended back to Adam in the twelfth century as part of the formation of the pseudo-Rhodri Mawr recension. This process drew upon several major sources, including early medieval Irish and Brittonic pseudo-history, Geoffrey of Monmouth's De gestis Britonum, and Trojan and biblical mythology. The second section addresses the changes made to the Gwynedd pedigree during the rule of Llywelyn ap Gruffudd, prince of Gwynedd (d. 1282), as attested in Exeter 3514 and the Mostyn genealogies. It is argued that these changes formed part of a deliberate response to Edward I's adoption of Geoffrey of Monmouth's historical narrative as a means to legitimise his imperial pretensions within Britain. The third section considers some other genealogies in Exeter 3514, a Latin manuscript of the late thirteenth century, in further detail. It is observed that these genealogies exercised a decisive influence on texts as diverse as Y Bibyl Ynghymraec, the Icelandic langfeðgatal and English royal genealogies of the fifteenth century. The fourth section turns to the genealogies of the Mortimer family, which drew upon independent sources of Welsh origin. Finally, the fifth section examines the final stage in the development of the ancestral Beli Mawr pedigree within Wales, when Geoffrey of Monmouth's prehistoric narrative was integrated into it more thoroughly. This development may be attributable to Gutun Owain, working in support of the new Tudor dynasty.