Crabtree G R, Comeau C M, Fowlkes D M, Fornace A J, Malley J D, Kant J A
J Mol Biol. 1985 Sep 5;185(1):1-19. doi: 10.1016/0022-2836(85)90179-2.
Chromosomal linkage as well as sequence homologies provide unequivocal evidence that the genes for the alpha, beta and gamma chains of fibrinogen arose by successive duplication of a single ancestral gene. Yet, when the three fibrinogen chains are aligned by amino acid homology, the positions of intervening sequences coincide at only two positions for all three chains. While one additional intron occurs at a homologous site in the beta and gamma chains, none of the positions of the remaining 11 introns in the three genes is shared. This arrangement of introns in the three fibrinogen genes suggests that either introns were selectively lost, implying that there is essential information in the retained introns, or the common introns were present in the ancestral fibrinogen gene and introns have been randomly inserted since the triplication of the original gene. The more likely possibility of selective loss of introns implies that the ancestral gene, as it existed about one billion years ago, must have been composed of numerous small exons.
染色体连锁以及序列同源性提供了明确的证据，表明纤维蛋白原的α、β和γ链的基因是通过单个祖先基因的连续复制产生的。然而，当三条纤维蛋白原链按氨基酸同源性排列时，所有三条链的间隔序列位置仅在两个位置重合。虽然在β链和γ链的同源位点出现了一个额外的内含子，但三个基因中其余11个内含子的位置没有一个是共享的。这三个纤维蛋白原基因中内含子的这种排列表明，要么内含子被选择性地丢失了，这意味着保留的内含子中有重要信息，要么共同的内含子存在于祖先纤维蛋白原基因中，并且自原始基因三倍化以来内含子被随机插入。内含子选择性丢失的更可能可能性意味着，大约10亿年前存在的祖先基因一定是由许多小外显子组成的。
