Structural organization of the beta-globin locus of B-haplotype sheep

Goats and some sheep synthesize a juvenile hemoglobin, Hb C (alpha 2 betaC2), at birth and produce this hemoglobin exclusively during severe anemia.Sheep that synthesize this juvenile hemoglobin are of the A haplotype.Other sheep, belonging to a separate group, the B haplotype, do notsynthesize hemoglobin C and during anemia continue to produce their adulthemoglobin. To understand the basis for this difference we have determinedthe structural organization of the beta- globin locus of B-type sheep byconstructing and isolating overlapping genomic clones. These clones haveallowed us to establish the linkage map 5' epsilon I-epsilon II-psi betaI-beta B-epsilon III-epsilon IV- psi beta II-beta F3' in this haplotype.Thus, B sheep lack four genes, including the BC gene, and have only eightgenes, compared with the 12 found in the goat globin locus. The goatbeta-globin locus is as follows: 5' epsilon I-epsilon II-psi beta X-betaC-epsilon III-epsilon IV-psi beta Z-beta A-epsilon V-epsilon VI-psi betaY-beta F3'. Southern blot analysis of A-type sheep reveals that theseanimals have a beta- globin locus similar to that of goat, i.e., 12 globingenes. Thus, the beta-globin locus of B-haplotype sheep resembles that ofcows and may have retained the duplicated locus of the ancestor of cows andsheep. Alternatively, the B-sheep locus arrangement may be the result of adeletion of a four-gene set from the triplicated locus.