1. A Negro family is described in which several individuals exhibit either the manifestations of thalassemia, or of the uncomplicated hemoglobin C trait; in two members the combined occurrence of these two anomalies is demonstrable. This combination is designated as hemoglobin C—thalassemia disease.

2. Hemoglobin C—thalassemia disease manifests itself in these two patients as a microcytic erythrocytosis. The red cells reveal a low MCV and low MCH, but a normal MCHC. About 45 per cent of the erythrocytes appear as target cells in the film. The amount of hemoglobin C in the hemolysates was about 75 per cent, the remainder being composed of A hemoglobin, and in one instance also of a small quantity of F hemoglobin. This is analogous to the results of the hemoglobin analyses in sickle cell—thalassemia disease, where 60 to 80 per cent of the pathologic hemoglobin S are found, though these individuals are heterozygous for both the pathologic hemoglobin and the thalassemia genes. The hypothesis is advanced that the thalassemia gene modifies (enhances) the expressivity of the gene for the pathologic pigment.

3. In genetic studies of families with thalassemia, hemoglobin analyses represent a necessary requirement. It is now established that thalassemia, as well as disorders associated with hemoglobin C, reveals a tendency to leptocytosis and thus may show erythrocytes with increased osmotic resistance.

4. The segregation of the thalassemia gene and of the genes for pathologic hemoglobins take place independently of each other. These genes are not allelomorphs.

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