20* MENDELISM 



crossed with a red oval-pollened plant, the blue, 

 instead of being coupled with the oval pollen in the 

 gametes of the heterozygote, is ' repelled ' by it, and 

 the great majority of the gametes will bear the blue- 

 round or red-oval characters. This is obviously only 

 another form of coupling, in which a dominant char- 

 acter is coupled with a recessive instead of two 

 dominants being coupled together, and the general 

 law of coupling (or as the Americans call it, linkage) 

 may be expressed thus: In the case of characters 

 which show gametic coupling, those factors which are 

 associated together in the homozygotes used as parents 

 tend to be associated together in the gametes produced 

 by the double (or multiple) heterozygote offspring. 

 The question of coupling of Mendelian characters with 

 a sex-determining factor (' sex-limited inheritance ') 

 will be mentioned in Chapter IX.] 



The second class of complications that we have to 

 deal with although the term complication may be 

 to a certain extent justified in connection with it 

 does not involve any exception to Mendel's law of 

 segregation. The phenomenon of so-called reversion 

 on crossing has long been familiar to biologists. Its 

 meaning, however, was totally obscure, and even the 

 Mendelian was at first unable to offer any explanation. 

 ThC phenomenon consists in the appearance, in the 

 offspring of a cross, of a character which was not 

 visibly present in either parent, and in many cases this 

 character can properly be regarded as ancestral it is 

 a character which has been lost by both parents in 

 the course of their divergent evolution from a common 



