Triplicated alpha-globin loci in humans.

Abstract
Individuals (12) who are heterozygous for a chromosome with 3 .alpha.-globin genes were identified. The presence of the 3rd .alpha.-globin locus was determined by restriction endonuclease digestion and hybridization with .alpha.-globin c[complementary]DNA probes. The 3 .alpha.-globin loci resided in an elongated fragment on digestion with EcoRI, BamHI and XbaI, and the 3rd locus was present in an additional 3.7-kilobase fragment on digestion with HpaI, SacI and BglII. The locations of the restriction sites are compatible with a nonhomologous crossover mechanism producing the triple .alpha. loci. The frequency of the triple .alpha. loci was 0.0036 in American blacks and < 0.004 in Sardinians, but was higher in Greek Cypriots (0.05). Individuals with 5 .alpha.-globin (.alpha..alpha./.alpha..alpha..alpha.) apparently showed no clinical or hematologic abnormalities.