CEOSSING OVER 



93 



spines. A list of these classes with the expectation based 

 on the results from a single experiment is given below. 



Non-crossover 

 wmf 23.2 



W M F2S.2 



Double crossover 

 in both I "^ ^/ 3.3 



regions 



W m F3.3 



Single crossover 



in 1st ( ^ ^^ ^ 13-2 



^^g^^^ ]w mf 13.2 



in 2nd f ^ ^" ^ ^-^ 

 region \^, j^jj 6 7 



Inasmuch as this subject and certain peculiarities in 

 the results can be better understood after the evidence for 



w 



M 



/ 



/ 



Fig. 38. — Scheme to illustrate double crossing over between white and forked. The gene 

 for miniature standing between furnishes the evidence. 



the linear order of the genes has been discussed, I shall 

 not press further the discussion here. It should be pointed 

 out, however, that the question of crossing over involves 

 more than the independent action of the pairs in the cases 

 so far considered; for, as will be shown later, when cross- 

 ing over takes place not single genes but great groups of 

 genes are involved. Tliis block effect, as it may be called, 



