The proportion of hybrid heterodimers in homozygous or doubly heterozygous beta chain variant hemoglobinopathies associated with alpha chain hemoglobin variants.

Four alpha genes exist on chromosome 16, but one or more of these genes can be deleted in association with Hemoglobin (Hb)G-Philadelphia in cis to alpha-thalassemia-2 in African-Americans. Therefore, the proportion of HbG-Philadelphia in HbG heterozygotes is trimodal at about 25% for alphaGalpha/alpha alpha, 33% for alphaG-/alpha alpha, and 50% for alphaG-/alpha alpha in patients with HbA. Those who are homozygous or doubly heterozygous for beta chain variants (betaX2 or betaXbetaY) have neither HbA nor the alpha chain variant (alphaX2 betaA2), but have hybrid heterodimers (alphaX2 betaX2). The proportion of hybrid heterodimers here should also be trimodal mirroring alpha gene status. Eleven patients were identified: 4 with Hb SSG, 3 with Hb SCG, and 1 each with Hb OCG, HbSSMontgomery, HbSSChicago, and HbSSBourmedes. Heterodimer proportions were: 43.3 +/- 1.5, 33.5 +/- 2.3, and 15.8 +/- 1.1% for 2, 3, and 4 respective alpha genes which had been studied in 8/11 of the patients (r = 0.98), implying that the prime determinant of the proportion of hybrid heterodimers in this patient group is the number of functional alpha genes.