据英国《新科学家》网站近日报道，美国科学家绘制出了目前世界上最先进的人类基因图谱，该图谱能帮助更准确地识别某些影响特定人群的遗传病根源。
这一基因图谱发现，西非人后裔的基因中有着欧洲人后裔所没有的基因重组的高发地带，这有可能是导致特定人种先天性疾病诸如贫血症的遗传学原因。当然，该基因图谱也在欧洲人后裔的遗传疾病研究中起了作用。
该基因图谱帮助识别遗传病根源的原理在于：人类单个正常受精卵中有两条同样的染色体，一条来自父方，一条来自母方。性细胞减数第一次分裂末期，亲本的这两条同源染色体分离，染色单体之间发生交叉互换。这一过程称为基因重组（此处指非等位基因自由组合，并且只发生在特定基因组的交叉地带）。基因重组过程中，有可能发生错误—基因序列的某个片段也许会异常缺失或错位。这样的错误往往会导致遗传病。
这一基因图谱由哈佛大学医学院的大卫·赖克和他的同事们通力完成。他们设计了新的基因演算法，通过对大约3万名非洲裔美国人的基因数据分析后，鉴定出约210万个发生基因重组的交叉地带。在此之前，世界上最精准的基因图谱基于1.5万名冰岛父母和其子女的数据，展示出50万处基因组交叉地带。
赖克研究团队正是利用了实验志愿者的混血血统，才发现了更多的基因组交叉地带。因为，通常来说，非洲裔美国人有百分之八十的西非基因和百分之二十的欧洲基因，这使他们染色体组的“非纯血统”基因片段长且完整。科学家们能通过寻找被西非基因片段“做上记号”的欧洲基因片段来精确定位基因组交叉地带，反之亦然。
研究人员对比新基因图谱与早先仅含有欧洲基因的基因图谱发现，西非基因中大约有2500个活跃的基因重组高发地带；而欧洲基因中这些地带则较为安静。这说明，这2500个基因重组高发地带为拥有西非基因的人群所特有，因而，也可能成为这部分人群特有遗传病的根源。
“我们有一些关于基因重组的新发现。”赖克说，“那就是，我们猜想基因重组率在每个人种中都是相同的，这也许对基因图谱研究和基因发现意义重大。”
生物探索推荐英文原文：
Most advanced genetic map may pinpoint diseases
People of west African ancestry have hotspots of gene recombination that are not seen in people of European descent. Such hotspots may be linked to genetic errors that contribute to congenital diseases like anaemia, so the finding could help identify the origin of diseases that particularly affect these people.
The finding comes from the most advanced map of the genome to date, which could also help in the study of genetic diseases in those with European ancestry.
Every normal body cell contains two copies of each chromosome, one inherited from each parent. During meiosis – the type of cell division that produces sperm and eggs – corresponding chromosomes from the individual's mother and father exchange fragments of DNA so that the resulting sperm or eggs contain genetic material from both parents. This process is called recombination, and happens at certain "crossover" sites of the genome.
Recombination can go wrong, however – a fragment of genetic code can be accidentally deleted or inserted where it does not belong. Such errors are often linked to genetic diseases.
Mixed ancestry
David Reich of Harvard University and his colleagues devised algorithms to analyse genetic data collected from nearly 30,000 African Americans and identified around 2.1 million crossovers where recombination occurred.
Before Reich's work, the most accurate genetic map was based on data from 15,000 Icelandic parents and their children, which revealed 500,000 crossovers.
Reich's team took advantage of the mixed ancestry of their participants to locate a higher number of crossovers. That's because the average African American has about 80 per cent west African ancestry and 20 per cent European ancestry, producing a genome with long an unbroken segments of either African or European ancestry. Reich's team could pinpoint crossovers by looking for segments of European ancestry that had been punctuated by a fragment of west African DNA, or vice versa.
African hotspots
When the researchers compared their new map with earlier maps that covered solely European genetics, they found important differences at a fine scale – namely about 2500 recombination hotspots that are usually active in people of west African ancestry but nearly always inactive in Europeans.
The 2500 recombination hotspots unique to people of west African ancestry may be associated with genetic diseases found only in such people.
"We found something new about recombination," says Reich. "We had somewhat naively assumed recombination rates are identical across humans. This is a fantastic new resource useful for gene mapping and gene discovery."
