280 



AN INTRODUCTION TO MODERN GENETICS 



into a Frederiksborg type. Woltereck suggests that this difference in 

 fixity of type is because the Nemi race was only exposed to the Nemi 

 conditions for something like five hundred generations, while the 

 Esrom race had probably been inhabiting that lake for a period cover- 

 ing some sixty thousand generations. But it is still doubtful whether the 

 difference in the time of exposure has acted by fixing an originally 

 labile cytoplasmic modification rather than in the more conventional 

 way of giving time for selection to build up in L. Esrom a race with its 

 characteristics controlled by genes. 



3. Maternal Effect 



Several cases are known in which adult characters clearly depend on 

 the nature of the cytoplasm of the egg, and this again is under the 



(fA(L 



(^Aa 



a aa 



WQ 





m 



1st Larval Instar 



1 1 1 



-=o i. V. 



Last Larval Instar 



Fig. 126. Maternal Effect in Ephestia. — Diagram of the ocelli in the first and 

 last larval Instars of the F1 from the cross aa x Aa. Note that if the cross is made 

 with Aa as the female parent the F1 are at first all alike, but that the ocelli of 

 the aa animals gradually become pale as the initial store of pigment becomes 

 exhausted. The upper row of figures shows the whole head of the larva, the 

 lower the ocelli only. 



(From KiJhn.) 



direct control of the genes in the mother. A well-known example is the 

 inheritance of direction of coiling in Limnea, which is discussed in 

 another connection on p. 191. Another very beautiful example has been 

 described by Kiihn^ in Ephestia. The gene A causes a more intensive 

 pigmentation of the larval ocelli (and also greater pigmentation in many 

 other regions). In a cross aa x Aa segregation is quite normal if the 

 mother is aa, but if the cross is made the other way, the progeny of the 



^ Ktihn 1936. 



