Abnormal hemoglobins; clinical disorders resulting from various combinations.

Within the past few years it has been noted that abnormal types of hemoglobin found in certain persons are associated with definite clinical disorders. At least four different varieties of sickle cell anemia are now recognized, three of them being heterozygous and one homozygous. When the gene for sickling is represented once, the person is asymptomatic and is said to have "sickle cell trait." However, when the sickle cell trait is present in combination with certain other hemoglobin abnormalities such as hemoglobin C or D or with thalassemia trait, symptomatic clinical disease results. The homozygous condition, in which two genes for hemoglobin C are present in the same person, has been observed in a few instances. A similar condition as regards hemoglobin D has not as yet been recognized.