A severe form of the genetic disorder thalassemia in which all four alpha chain polypeptides making up the hemoglobin molecule (the protein which transports oxygen in the body) are missing due to a defect in the gene which codes for them. As a result, affected individuals die at or before birth.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.