The result of a marriage between an individual with delta thalassemia and carrier of the Persistent High Hb-F gene with 13 siblings is discussed. The incidence, genetic and hematologic manifestations of both disorders are presented. The complete absence of Hb-A2 in 3 siblings from this critical marriage tend to support the hypothesis of Gerald and associates that basic cis-trons are responsible for final protein amino acid sequence. This is the 1st known example of such an interaction.