The common two-gene mutations (--SEA/aa) and (--FIL/aa) are of particular importance due to the risk
of causing Hb H disease when inherited along with a single-gene deletion or resulting in a-thalassemia
major (Hb Bart’s hydrops fetalis) when inherited from both parents. Based on the frequency of the
--SEA/aa carrier state, the incidence of Hb Bart’s hydrops fetalis is expected to be between 0.5 and five
per 1000 births, and Hb H disease between four and 20 per 1000 births[1]. The combination of one of
these common two-gene deletions with a nondeletion form of a-thalassemia, in particular the Constant
Spring allele, is also common. The Constant Spring results from a mutation in the “stop codon” (a142,
Term-->Gln, TAA-->CAA in a2), which leads to the insertion of the amino-acid glutamine instead of
termination of the chain synthesis, causing an elongated a-chain. This results in a greater degree of globin
imbalance and a preferential binding of the mutated globin chain to the red cell membrane, causing its
rigidity and instability. The cells are overhydrated, which manifests in a higher mean corpuscular volume
(MCV) than seen in Hb H disease (66.1–68.0 vs. 57.7–64.8 fl).