CHAPTER VIII 



THE INTERPEETATION OF REVERSION 



For instances of reversion which may be explained 

 in the light of Mendelian facts of inheritance we can 

 again draw on the characters of Pisum. One of the 

 characters with which Mendel experimented was the 

 colour of the seed-coat, which was either grey 

 (dominant) or white (recessive). The grey seed-coat 

 might, or might not, be marked with the purple 

 spots which contributed to the subject matter of 

 the last chapter. We are now concerned with the 

 grey seed-coat which is stated to be destitute of the 

 purple spot. As a matter of fact, purple spots are 

 present, but they are very faint and scarcely dis- 

 cernible. So that the difference between the two 

 kinds of greys is not that one has, and the other has 

 not, purple spots, but that one has pronounced purple 

 spots (which have already been seen in one of the 

 parents of the cross described in the last chapter, 

 Fig. 26) and the other scarcely discernible ones (which 

 cannot be seen in the pea to the top left of Fig. 27). 

 The latter will be referred to simply as grey to avoid 

 circumlocution ; the former as grey with purple 

 spot. We are now concerned with the spotless 

 grey (Fig. 27, top left), and with the pure white, 



which is shown at the top right of Fig. 27. When 



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