Li W H, Gojobori T
Center for Demographic and Population Genetics, University of Texas, Houston 77025.
Mol Biol Evol. 1983 Dec;1(1):94-108. doi: 10.1093/oxfordjournals.molbev.a040306.
Statistical analyses of DNA sequences of globin genes (beta A, beta C, and gamma) from goat and sheep (including new sequence information for the second intron of sheep beta A and gamma, kindly provided by A. Davis and A. W. Nienhuis) indicate that the rates of nonsynonymous substitution in these genes have been greatly accelerated following the gene duplication separating gamma and the ancestor of beta A and beta C and the gene duplication separating beta A and beta C. In both cases the acceleration was apparently due to relaxation of purifying selection (functional constraints) rather than advantageous mutations because acceleration occurred only in less important parts of the beta globin chain. The rates of nonsynonymous substitution in these genes are estimated to be about 2.3 x 10(-9) per site per year, which is three times higher than that for the divergence between human beta and mouse beta major globin genes. Our analyses further suggest that the rate of synonymous substitution in functional genes and the rate of substitution in pseudogenes are approximately equal and are between 2.8 x 10(-9) and 5.0 x 10(-9) and that the rate of substitution in introns is about 3.0 x 10(-9). The divergence time between beta A and beta C and that between gamma and the beta A-beta C pair are about 12 and 30 million years, respectively. The proportion of transition mutations is estimated to be 64%, two times higher than expected under random mutation but considerably lower than the 96% estimated for animal mitochondrial DNA.
对山羊和绵羊的珠蛋白基因（βA、βC和γ）的DNA序列进行统计分析（包括由A. 戴维斯和A. W. 尼恩huis慷慨提供的绵羊βA和γ第二个内含子的新序列信息）表明，在γ与βA和βC的祖先分离的基因复制以及βA和βC分离的基因复制之后，这些基因中的非同义替换率大大加快。在这两种情况下，加速显然是由于纯化选择（功能限制）的放松，而不是有利突变，因为加速仅发生在β珠蛋白链不太重要的部分。这些基因中的非同义替换率估计约为每年每个位点2.3×10⁻⁹，这比人类β和小鼠β主要珠蛋白基因之间的分歧率高三倍。我们的分析进一步表明，功能基因中的同义替换率和假基因中的替换率大致相等，在2.8×10⁻⁹和5.0×10⁻⁹之间，内含子中的替换率约为3.0×10⁻⁹。βA和βC之间以及γ与βA - βC对之间的分歧时间分别约为1200万年和3000万年。转换突变的比例估计为64%，比随机突变预期的高两倍，但远低于动物线粒体DNA估计的96%。
