Curriculum · Hematology
Alpha-thalassaemia
What it is
Alpha thalassemia is caused by a defect, or defects, in the alpha genes. A normal person carries four functional alpha genes, and the number that are still functional decides which picture appears:
- Normal: 4 functional alpha genes
- Silent alpha thalassemia: 3 functional alpha genes
- Alpha thalassemia trait, homozygous alpha+: 2 functional alpha genes
- Alpha thalassemia trait, heterozygous for alpha0: 2 functional alpha genes
- HbH disease: 1 functional alpha gene
- Hb Bart's Hydrops Fetalis Syndrome, which is lethal: 0 functional alpha genes
Two different states both leave two functional alpha genes and both are called alpha thalassemia trait: homozygous alpha+, and heterozygous for alpha0.
The same states are written a second time as genotypes, with the picture beside each. A single gene deletion gives normal or only slightly hypochromic red cells. A two gene deletion, in either of its two forms, is alpha thalassemia trait and gives a mild microcytic anemia. Three genes lost is HbH disease, a microcytic anemia that occasionally requires transfusion. Four genes lost is Hb Bart's hydrops fetalis syndrome. So it is the count of what is lost, and not which of the two forms it takes, that sets the picture.