[请教]Y染色体的突变能够提供这么多的确定性信息吗？

狸狸 · Post by 狸狸 » 2006-05-16 21:03

我还是很糊涂的是突变的问题，突变应该是孤立事件吧？前面提到的多长时间2%-4%，是包括各种各样的突变呢，还是之前的突变在进化过程中占到优势，积累下来的呢？
我的理解是：坏的突变会自然被剪枝，好的突变可能会逐步扩大并且加强。假如说大象的长鼻子是这样进化出来的，那是否可能是来源于第一次的突变导致鼻子长了一厘米，过了几千年长一厘米的象群里又出现一个长一厘米的突变，现在我们看到的就是好多次突变淘汰之后累加的结果。这样的概念对吗？

Boubouka · Post by **Boubouka** » 2006-05-16 22:24

Cool!

Jun · Post by **Jun** » 2006-05-17 6:45

我的理解是：坏的突变会自然被剪枝，好的突变可能会逐步扩大并且加强。假如说大象的长鼻子是这样进化出来的，那是否可能是来源于第一次的突变导致鼻子长了一厘米，过了几千年长一厘米的象群里又出现一个长一厘米的突变，现在我们看到的就是好多次突变淘汰之后累加的结果。这样的概念对吗？

No. Not really.

Can't write an explanation now, but will write more later.

Jun · Post by **Jun** » 2006-05-17 7:15

Maomitou is right. There are some functional genes on Y chromosome and a lot of junk (neutral mutation). I was mistaken on that point.

Not a lot of defects. Any significant negative mutation (ie, causing harm) would be quickly eliminated because there is no "paring" for Y chromosome like all the other chromosomes (including XX in women) during fertilization. For paired chromosomes, if you have a defect, you still have a back-up.

silkworm · Post by **silkworm** » 2006-05-17 8:33

狸狸 wrote:我还是很糊涂的是突变的问题，突变应该是孤立事件吧？前面提到的多长时间2%-4%，是包括各种各样的突变呢，还是之前的突变在进化过程中占到优势，积累下来的呢？

突变对于个体是突发孤立事件，对于群体+上百万年就是连续发生的事件了。当然会有一些时段，因为环境因素（瞎说一个，譬如冰川时代来了什么的）突变速率会改变，但是分子钟是算个平均值，应该影响不大。

至于积累下来的是“好”突变还是“坏”突变，我感觉坏突变早让那个物种死翘翘了，但是好象又不是在查好突变的积累，应该是些不致死也没好处的中性突变（neutral mutation）。

比如有些组做Y染色体的追溯，看的是某个基因的intron（中文？内含子？）的突变。intron基本上是翻译（translation）不出蛋白质的，也就是说稍微突变了点儿，也无伤大雅。

森林的火焰 · Post by **森林的火焰** » 2006-05-17 8:35

好象就是要选择中性突变才能用突变速估算在进化上分离的时间。如果是严肃的突变，环境因素的影响太大，往往追不回以前这个基因是什么样子的了。

silkworm · Post by **silkworm** » 2006-05-17 20:45

Look at this one. Isn't it cool?

Associated Press
DNA Study Maps Human-Chimp Split
By MATT CRENSON , 05.17.2006, 04:57 PM
One of the most detailed comparisons yet of human and chimp DNA shows that the split between the two species was a long, messy affair that may even have featured an unusual evolutionary version of breakup sex.

Previous genetic research has shown that chimpanzees and humans are sister species, having split off from a common ancestor about 7 million years ago. The new study goes farther by looking at approximately 800 times more DNA than earlier efforts.

That additional data make it possible to determine not just when, but how the split happened.

"For the first time we're able to see the details written out in the DNA," said Eric Lander, one of the collaborators on the study. "What they tell us at the least is that the human-chimp speciation was very unusual."

Unusual, indeed. The researchers, from the Broad Institute of MIT and Harvard, propose that humans and chimpanzees first split up about 10 million years ago. Then, after evolving in different directions for about 4 million years, they got back together for a brief fling that produced a third, hybrid population with characteristics of both lines.

That genetic collaboration then gave rise to two separate branches - one leading to humans and the other to chimps.

The work has inspired both admiration and skepticism. Many paleontologists have a hard time believing that some of the fossil humans that are known to have lived during that era could have been pairing up with apes.

"It's a totally cool and extremely clever analysis," said Daniel Lieberman, a professor of biological anthropology at Harvard who wasn't involved in the study. "My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates - not to put it too crudely."

Past studies that compared human and chimp DNA could only average the differences between a limited number of spots in their genetic codes to come up with a single date for the split, rather than a span of years. The genius of the new study is that it breaks the genetic code into pieces and then looks at each section individually.

Surprisingly, some genes differ so much between the two species that they must not have been mixing for the past 10 million years. But others are similar enough that they appear to have been in contact no more than 6.3 million years ago.

That finding, and some details about which particular genes split when, led the study's authors to propose their controversial scenario.

The new data also suggest the final human-chimp split was much more recent than the 7 million-year date that fossils and previous studies indicate - certainly no earlier than 6.3 million years ago, and more likely in the neighborhood of 5.4 million years.