CHAPTER XI 



COUPLING AND REPULSION 



Although different factors may act together to 

 produce specific results in the zygote through their 

 interaction, yet in all the cases we have hitherto 

 considered, the heredity of each of the different factors 

 is entirely independent. The interaction of the 

 factors affects the characters of the zygote, but makes 

 no difference to the distribution of the separate 

 factors, which is always in strict accordance with 

 the ordinary Mendelian scheme. Each factor in 

 this respect behaves as though the other were not 

 present. 



A number of cases, however, have now been 

 worked out in which the distribution of the different 

 factors to the gametes is affected by their simultane- 

 ous presence in the zygote. An interesting example 

 occurs in the sweet-pea, in which the phenomenon 

 was originally discovered. It has already been 

 pointed out in Chapter VIII. that, in respect of 

 colour, sweet-peas may be divided into two classes, 

 viz. purples and reds. Purple behaves as a simple 

 dominant to red. The difference between any 

 given purple and the corresponding red is that the 



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