Beta-globin gene cluster haplotypes in Hb C heterozygotes.

Previous work has suggested that the beta C mutation arose on a relatively rare beta A chromosome with a unique haplotype, and these data have been used to postulate a single origin of the beta C allele. Most characteristic of this haplotype has been the absence of a Hpa I recognition site 3' to the beta-globin gene. We studied 40 patients with Hb SC, 11 with Hb AC, and one with Hb CC. Complete haplotyping was possible on 44 beta C alleles and 3' haplotyping was possible on an additional four beta C alleles. Our results support the predominence of a unique beta C haplotype which is characterized by the absence of the Hpa I site. However, one patient who was homozygous for Hb C had one beta C allele with an atypical haplotype which contained the Hpa I site. It is unclear at this time whether or not this represents a second origin of the beta C mutation.