While loss‐of‐function mutations in G_sα are invariably associated with the short stature and brachydactyly of Albright hereditary osteodystrophy (AHO), the association with hormone resistance (to parathyroid hormone and thyrotropin) typical of pseudohypoparathyroidism type Ia (PHP‐Ia) is much more variable. Observational studies and DNA polymorphism analysis suggest that maternal transmission of the G_sα mutation may be required for full expression of clinical hormone resistance. To test this hypothesis, we studied transmission of a frameshift mutation in G_sα through three generations of a pedigree affected by AHO and PHP‐Ia. While all family members carrying this loss‐of‐function mutation in one G_sα allele had AHO, neither the presence of the mutation nor the degree of reduction of erythrocyte G_sα bioactivity allowed prediction of phenotype (AHO alone versus AHO and PHP‐Ia). Paternal transmission of the mutation (from the patriarch of the first generation to three members of the second generation) was not associated with concurrent PHP‐Ia, but maternal transmission (from two women in the second generation to four children in the third generation) was invariably associated with PHP‐Ia. No expansion of an upstream short CCG nucleotide repeat region was detected, nor was there evidence of uniparental disomy by polymorphism analysis. This report, the first to document the effects across three generations of both paternal and maternal transmission of a specific G_sα mutation, strongly supports the hypothesis that a maternal factor determines full expression of G_sα dysfunction as PHP‐Ia. Am. J. Med. Genet. 77:261–267, 1998. © 1998 Wiley‐Liss, Inc.