The concept of genetic penetrance, "the frequency of manifestation of a genetic factor," was introduced by Timofeef-Ressovsky (Naturwissenschaften 19:493,1931). Incomplete penetrance has been used to explain the absence of phenotypic expression when otherwise anticipated. Studies of Embryonic Chick Edema, ECE (Poultry Sci. 77(suppl. 1):69, 1998) have been conducted in order to determine the origins for incomplete penetrance of this disorder. ECE was originally reported as the expression of two autosomal recessive loci with incomplete penetrance. Pedigreed inter se mating of ECE individuals have resulted in familial incidences ranging for 0 to 100% with a mean of 48.2% in the most recent generation selected. With consideration of a third contributing locus, current data and 2 sets of previous data were evaluated. Heterogeneity and pooled chi square tests when applied to the data sets support the hypothesis that ECE was the result of two completely dominant loci and one homozygous recessive locus. / Graduation date: 2004