Two different mutations in codon 68 are observed in Hb G-Philadelphia heterozygotes. 
Through sequencing of amplified DNA containing the appropriate alpha-globin genes we have identified the base substitution leading to the formation of Hb G-Philadelphia [alpha 68(E17)Asn Lys].  Three subjects (approximately 25% Hb G) had an ACC AAA change at codon 68 of the alpha 2-globin gene; the chromosome with this mutation carried two alpha genes (alpha alpha).  Six subjects (approximately 33% Hb G) had an AAC AAG change at the same codon of the alpha 2 alpha 1 hybrid gene on a chromosome with the 3.7 kb deletion (-alpha 3.7).  These results indicate two independent mutations which likely occurred in different populations; increase in the level of Hb G is primarily dependent upon the loss of one or more alpha-globin genes.