A group of human anemias due to imbalance in the ratio of alpha and/or beta hemoglobin subunits. Since there are four alpha genes per genome, deletions (commonly produced by unequal crossing over) can result in an individual having any number of alpha genes from zero to four. The complete absence of alpha genes produces hydrops fetalis (q.v.). With only one alpha gene, excess beta chains form a tetramer (β4), resulting in hemoglobin H disease. Individuals with two or three alpha genes are almost indistinguishable from normal. Epidemiological studies have shown that individuals with alpha thalassemia trait (--/aa or -a/-a) are more resistant to malaria than aa/aa individuals. Incomplete beta chains can be produced by nonsense codons. Deletions in beta genes are commonly produced by unequal crossing over, as are the hybrid chains containing δ and β segments (Hb Lepore) or Aγ and β segments (Hb Kenya). Beta thalassemia (also called Cooley anemia) is a hemoglobinopathy in which few functional beta globin chains are made. A point mutation, within an intron that alters the cutting and splicing signal, causes an extra piece of intron RNA to be present in processed mRNA; the extra piece shifts the reading frame and causes translation to stop prematurely, yielding a truncated and nonfunctional beta globin molecule. See Chronology, 1976, Kan et al.; 1986, Costantini et al.; Desferal, hemoglobin fusion genes, hemoglobin homotetramers. http://www.thalassemia.org
Subjects: Genetics and Genomics.