Exceptionally high prolificacy in the Cambridge and Belclare breeds has been shown to be attributable to point mutations in genes (BMP15 and GDF9) that code for oocyte derived growth factors which have profound effects on ovarian follicle development. The BMP15 gene is X linked while GDF9 is autosomal (chromosome 5). Two different BMP15 mutations are present in the Belclare population (FecXG and FecXB) but only one of these (FecXG) was found in the Cambridge while the GDF9 mutation (FecGH) was common to both breeds. The Inverdale and Hanna mutations in NZ Romney sheep (Davis et al. 2001) are point mutations in BMP15 but involve different codons from those involved in Cambridge and Belclare. It is of interest to know whether the mutations common to Cambridge and Belclare are likely to have had a common ancestral source or arose independently. The Lleyn breed was a common source of genetic material in the genesis of both breeds. Thus, the foundation ewes for the Cambridge included three Lleyn ewes (Owen,1976) while a set of 13 Lleyn sheep selected for high litter size was a major contributor to the Belclare breed (accounted for 50% of the genetic material at one stage; subsequently diluted to about 25%). The hypothesis is that the Lleyn was the likely source of the two mutations common to Belclare and Cambridge.