The Genetics and Molecular Basis of Alpha Thalassaemia in Association with Hb S in Jamaican Negroes

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Abstract
Seven Jamaican Negro families in whom the genes for .alpha.-thalassemia and the sickle cell mutation (.beta.s) were independently segregated were studied. Using a combination of techniques, 2 .alpha.-thalassemia phenotypes which resemble the severe (.alpha.-thalassemia 1) and mild (.alpha.-thalassemia 2) determinants previously described in Orientals were identified. This study permitted the clear correlation of the phenotype of .alpha.-thalassemia with the genotype in this population. Since in each family .alpha.-thalassemia was present in association with the gene for the sickle cell mutation, the proportion of Hb S in the peripheral blood of individuals with the .alpha..alpha./.alpha..alpha., -.alpha./.alpha..alpha. and -.alpha./-.alpha. genotype who are also heterozygous for the .beta.s mutation was determined. Genetic analysis in these families shows that in each case, subjects with the .alpha.-thalassemia 1 phenotype are homozygous for the .alpha.-thalassemia 2 defect (-.alpha./-.alpha.). No instances of the genotype --/.alpha..alpha. were found in this population, which may explain the rarity of the severe .alpha.-thalassemia syndromes in Jamaica. Restriction mapping data in the .alpha.-thalassemia 2 homozygotes from this population shows that the (-.alpha./) haplotype results from a deletion of one of the linked pair of .alpha.-globin genes and that this has probably arisen by an unequal crossover between non-homologous .alpha. genes.
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