A couple days ago the 60 Second Science podcast had an interesting story on dating medieval manuscripts. It turns out you can take DNA samples from the animal products in the pages. The hope is that the pages in undated manuscripts will have the same DNA signature as extant dated manuscripts, suggesting that the books were written around the same time.
The canonical example of the normal distribution given in textbooks is human heights. Measure the heights of a large sample of adult men and the numbers will follow a normal (Gaussian) distribution. The heights of women also follow a normal distribution. What textbooks never discuss is why heights should be normally distributed.
Why should heights be normally distributed? If height were a simple genetic characteristic, there would be two possibilities: short and tall, like Mendel’s peas that were either wrinkled or smooth but never semi-wrinkled. But height is not a simple characteristic. There are numerous genetic and environmental factors that influence height. When there are many independent factors that contribute to some phenomena, the end result may follow a Gaussian distribution due to the central limit theorem.
The normal distribution is a remarkably good model of heights for some purposes. It may be more interesting to look at where the model breaks down. See my next post, why heights are not normally distributed.
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Researchers recently discovered that identical twins are not genetically identical after all. They differ in the copy numbers of their genes. They have the same genes, but each may have different numbers of copies of certain genes.
Source: “Copy That” by Charles Q. Choi, Scientific American, May 2008.
A gene therapy developed at M. D. Anderson Cancer Center for head and neck cancer is the first such treatment to succeed in a phase III trial. See the press release for more details.
(Phase III studies are large, multi-institutional studies required for regulatory approval of new drugs.)