And most men manage to protect themselves from injury so far as is needed to keep both ears. Whereas trees cannot protect themselves form the loss of branches. It is very rare to see a completely unmutilated, and therefore completely regular tree. Mendels laws, according to which two types occur in a ratio of 1: 1 in some cases and 3 : 1 in others, are theoretically true if the processes of division of cell nuclei are quite regular, and if neither type is unfit so as to die off before counts are made. The first condition never holds, and the second probably never does. But the exceptions to the first condition are very rare. In one particular case a critical division goes wrong about one in ten thousand times. The effect of this on a 1 : 1 ratio or 3 ! 1 ratio could be detected only by counting several hundred million plants or animals. Differences in relative fitness are more important. But even so the Mendelian ratios are sometimes fulfilled with extreme accuracy, and are generally a good rough guide.
Jeffreys points out that in such cases it is often much better to stick to the theoretical law rather than the observed data. For example, if you are breeding silver foxes and a new colour variety occurs which, if crossed to the normal, gives 13 normal and 10 of the new colour, you are much more likely to get a ratio of about 1:1 than 13:10 if you go on with such matting, even though if you breed many thousands the 1: 1 ratio will not hold exactly. The mathematical theory which Jeffreys has developed concerning such cases is particularly beautiful, but can hardly be summarized here.
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