Patterns of polymorphism and linkage disequilibrium suggest independent origins of the human growth hormone gene cluster.

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Abstract
Six restriction fragment length polymorphisms (RFLP) detected in the human growth hormone-human chorionic somatomammotropin (hGH-hCS) gene cluster were studied in Mediterraneans, Northern Europeans, and American Blacks; the polymorphisms showed that, on the average, 1 of 500 bases in this cluster is variant. Haplotypes constructed for 4 of these RFLP display strong nonrandom associations. However, the strongest associations were between RFLP that are in homologous DNA rather than between the physically closest RFLP. From this and other evidence duplication of an ancestral hCS gene evidently occurred at least twice, the 2nd event being relatively recent. In other words, duplication of the hCS-L gene to produce the hCS-A gene occurred twice, so that hCS-A genes in humans may have independent origin. Chromosomes with absent hCS genes (leading to hCS deficiency) may represent the nonduplicated ancestral unit rather than gene deletions.