Sequence and evolutionary history of the length polymorphism in intron 1 of the human red photopigment gene.

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The length of intron 1 of the red pigment gene is dimorphic among Africans but not among Caucasians or Asians. This dimorphism was found to result from the presence or absence of a block of 1,284 bp comprised of three Alu elements and 328 bp of intervening unique-sequence DNA. This additional sequence in the "long" intron 1 of the red pigment gene was shown to have inserted into a fourth and older Alu element present in the "short" from of intron 1. Furthermore, the size and sequence of the "short" intron 1 of the red pigment gene is equivalent to that of the adjacent green pigment gene. The block containing the three Alu elements was not found in intron 1 of the red or green pigment genes of Old World monkeys and orangutans but was present in intron 1 of both the green and red pigment genes of gorillas and chimpanzees. The nucleotide sequence of this block in Old World primates and the estimated ages of the three elements suggest that their insertion occurred sequentially in the Old World monkey lineage prior to duplication of the ancestral X-chromosome-linked pigment gene. After gene duplication, deletion of the entire block containing the three Alu elements from one of the genes created the "short" intron variant. Unequal recombination between the adjacent and highly homologous red and green pigment genes may have resulted in the formation of the "short" intron variant of the other gene.