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【科普】Looking for roots in Africa? DNA search not easy
By Maggie Fox, Health and Science Editor
WASHINGTON (Reuters) - African-Americans hoping to use DNA to find their roots may have to look harder than previously thought, researchers said on Thursday in a study they said shows Africans are too genetically mixed to make tracing easy.
Several companies now offer to help Americans trace their African ancestry using mitochondrial DNA, which is passed from mother to daughter virtually unaltered.
"What's your tribe?" asks one. "Trace Your Roots Back in Time," offers another.
But biologist Bert Ely of the University of South Carolina and colleagues found that fewer than 10 percent of African-American mitochondrial DNA sequences that were analyzed can be matched to any single African ethnic group.
The news might disappoint people who cannot trace a long paper trail of ancestors. More than 11 million Africans were forcibly shipped to the Americas during the slave trade that peaked in the 17th and 18th centuries.
"The test that everybody does is the same test and it is all valid," Ely said in a telephone interview.
"It is just that some companies will over-interpret the data and give you the most likely result instead of all of the matches."
Working with colleagues at the University of Massachusetts and the University of Maryland, Ely looked at 3,700 mitochondrial DNA sequences from people in sub-Saharan Africa.
They also sampled African-Americans, including people who identify themselves as "Gullah" or "Geechee" and live along the coasts of South Carolina and Georgia. These groups have close cultural ties to Sierra Leone, including language, stories and crafts.
Because there is more genetic diversity among Africans than among people from any other continent and because humanity has been in Africa longer than anywhere else, Ely said the idea of being able to trace one's DNA to a certain tribe or place sounds logical.
"That would be true if everybody stayed put, but they have a lot of history of moving around," Ely said.
With the Gullahs and Geechees, it was possible the genetic links were more pure, but Ely's team did not find that. "The analysis does not show their mitochondrial DNA is different from a random sample of Africans," he said.
"It was unlikely because slaves were brought into Charleston from all over the western part of Africa," Ely added.
"Historical accounts indicate that virtually all enslaved Africans brought to North America came from either West or West Central Africa," Ely's team wrote in their report, published in the online journal BioMed Central Biology.
"The truth was we know perfectly well who the ancestors of modern African-Americans were, and they were people from all up and down the west coast of Africa," Ely said.
"What no one has really looked at is how often can someone trace their roots back to a single ethnic group back in Africa and there the answer came out to be not very often -- maybe 10 percent or less."
Ely's team is now using the genetic database to look for medical information, such as possible genes that cause a more aggressive form of breast cancer in U.S. blacks. 寻根非洲，DNA搜索并非易事
来自：Maggie Fox 《健康与科学》编辑
非裔美国人希望能通过DNA来寻根，可实现这个愿望之难出乎意料。星期二，研究者们表示，非洲人在遗传学上的复杂性令寻根之旅困难重重。
现在有许多公司利用线粒体DNA来帮非裔美国人寻找祖先，线粒体DNA保守地沿母系遗传。
——“你的部落是哪个？”
——“追溯你的根吧。”
但南卡罗来纳大学的生物学家Bert Ely和他的同事发现，经过分析的线粒体DNA序列能符合单一非洲部落的非裔美国人不到10％。
这条消息会令许多那些无法寻祖归宗的人大失所望。在17、18世纪奴隶买卖高峰时期，有一亿一千万非洲人被迫运至美国。
“每个人所做的测试都是一样，也都有效，”Ely在接受电话采访时说道。
“但是有些公司对数据解释不当，他们给出了最符合的结果并非完全匹配。”
Ely与麻省大学、马里兰大学的同事们一起查看了3,700个从撒哈拉以南地区人体中提取的线粒体DNA序列。
他们也从非裔美国人身上取样，包括那些认为自己是“嘎勒人”或“吉齐人”且住在南卡罗来纳州和乔治亚州的人们。这些人与Sierra Leone有着千丝万缕的文化渊源，包括语言、传说和手工艺等等。
由于非洲人比其他大陆的人有着更复杂的遗传多样性，也由于人类在非洲存在的时间比其他大陆久远得多，Ely认为通过人的DNA来确定部落和地区似乎是合乎逻辑的。
“如果每个人都居住固定，没有迁徙，上述可能究存在。但是人们的迁徙历史却是纷繁冗杂的。
嘎勒人和吉齐人的遗传联系有可能比较纯，但是Ely的研究人员发现事实并非如此。“经过分析，他们的线粒体DNA与任意一个非洲人的样本比较并无两样。“他说。
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作者:admin@医学,生命科学 2011-09-07 05:14
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