Pedigree in formation from livestock populations may be used to evaluate their genetic variability, the inbreeding of their individuals or the inbreeding depression on traits of interest. Three Iberian pig strains (Guadyerbas, Gamito y Torbiscal), sharing a common group of the animal founders, have been analyzed with this approach. The results evidence the genetic importance of a founder sow (Ruperta), with a proportional contribution to the populations stabilized in respective values 0.18, 0.29 and 0.09, being 4.04, 4.21 and 9.90 the mean number of surviving founder alleles in the respective last cohorts and 0.70, 1.07 and 0.77 those coming from Ruperta. The proportion of inbreeding due to this founder, measured over the litters born in the three strains, is 0.19, 0.33 and 0.10. The analysis of inbreeding depression on litter size in Torbiscal indicates a disminution of −0,13 liveborn piglets per each 10% increase of litter inbreeding, not being significant the effect of dam inbreeding. The effect on the litter size of the component of litter in breeding due to Ruperta was studied in the other strains, but the inferences obtained from the available data present a great uncertainity.