1. A Caucasian family is described in which, on the basis of clinical, hematologic and biochemical findings, it is postulated that the genes responsible for hemoglobins S and G and for the thalassemia defect are present.

2. On the basis of the study of this family, it is concluded that:

a. The genes responsible for hemoglobins G and S cannot be alleles.

b. The genes responsible for hemoglobin G and thalassemia cannot be alleles.

c. The absence of hemoglobin A in individuals heterozygous for two "hemoglobin genes" does not provide critical evidence concerning the allelic relations of such genes.

d. In this family, heterozygosity for the gene responsible for hemoglobin G results in an asymptomatic trait condition, in which some 40% of the hemoglobin is abnormal. When the gene responsible for G is combined with a hemoglobin S gene or a thalassemia gene, or both, the presence of hemoglobin G does not significantly alter the expression of these genes on their combinations. For example, an individual of the phenotype SG, whose hemoglobin contained no demonstrable A, was clinically a sickle cell trait, in that he showed no evidence of physiologic handicap.

e. Individuals heterozygous for both the G and thalassemia genes may show on electrophoresis only hemoglobin G. This illustrates the unreliability in some cases of diagnosing genotype on the basis of electrophoretic findings.

f. On the basis of these findings, hemoglobin G should probably be regarded as a normal variant of hemoglobin rather than as an abnormal type of hemoglobin.

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