Shen S H, Slightom J L, Smithies O
Cell. 1981 Oct;26(2 Pt 2):191-203. doi: 10.1016/0092-8674(81)90302-0.
The nucleotide sequence of 11.4 kilobase pairs (kb) of human DNA that includes the two fetal globin genes, G gamma and A gamma, shows that they are part of a 5 kb tandem duplication. A small segment of DNA occurs three times, on either side of and between the two duplicates. We present two models that account for these observations. One model is simple but requires the assumption of a preexisting repetitive element; the other is more complex but does not require the assumption of preexisting repeats. Over much of the 5 kb duplicated region, the present duplicate copies differ by an average of 14% of their bases, from which we calculate that the duplication was first formed about 34 million years ago. However, 1.5 kb of DNA are vitually identical in the two genes analyzed here, probably as a consequence of an intergenic exchange (gene conversion) that replaced part of the diverging A gamma gene with the corresponding part of the G gamma gene. This conversion took place around 1 million years ago. A sequence of repeated dinucleotides may be one signal involved in exchanges leading to this and similar conversions. Cycles of duplication, triplication, deletion and gene conversion on probably common in many multigene families.
一段包含两个胎儿珠蛋白基因Gγ和Aγ的11.4千碱基对（kb）人类DNA的核苷酸序列表明，它们是一个5 kb串联重复序列的一部分。一小段DNA出现了三次，分别位于两个重复序列的两侧和它们之间。我们提出了两种解释这些观察结果的模型。一种模型很简单，但需要假设存在一个预先存在的重复元件；另一种模型更复杂，但不需要假设预先存在的重复序列。在5 kb重复区域的大部分区域，目前的重复拷贝平均有14%的碱基不同，由此我们计算出该重复序列大约在3400万年前首次形成。然而，这里分析的两个基因中有1.5 kb的DNA实际上是相同的，这可能是基因间交换（基因转换）的结果，即用Gγ基因的相应部分替换了Aγ基因中部分差异较大的区域。这种转换发生在大约100万年前。重复二核苷酸序列可能是导致这种转换及类似转换的交换过程中涉及的一个信号。重复、三倍化、缺失和基因转换的循环可能在许多多基因家族中很常见。
