390 



INTRODUCTION TO CYTOLOGY 



this point would be prevented from crossing over at the same time, for the 

 reason that the twisting of the chromosomes about each other is not close 

 enough to allow two crossovers so near each other. Thus in Fig. 154, if 

 crossing over occurred between the two factor pairs Dd and Ee, breaking 

 the linkages between D and E and between d and e, there would be at the 

 same time no such break between Cc and Dd or between Ee and Ff, 

 since for mechanical reasons a second crossing over could not take place 

 at either of these points simultaneously with that between Dd and Ee. 

 Crossing over at one point would thus interfere with crossing over which 

 might otherwise occur at nearby points. The amount of 

 this interference would progressively decrease at points 

 farther and farther from the first crossover point, until 

 at a certain distance (measured by the length of the loops 

 usually formed by the twisting chromosomes), as atLM, 

 it would vanish entirely; here crossing over would occur 

 with its normal frequency irrespective of any crossover 



Muller has found that the characters behave accord- 

 ing to these expectations. If two characters have their 

 linkage broken in a certain percentage of cases, this 

 percentage is noticeably lowered if breaks in linkage 

 occur between two other characters having normally a 

 fairly close linkage with the first two. In other words, 

 one linkage break interferes with other linkage breaks 

 within the same linkage group; and the degree of this 

 interference varies from a high value in the case of a 

 closely linked series of characters to zero in the case of 

 characters very loosely linked. This, it is pointed out, 

 is just what should occur among characters represented 

 by a linear series of genes in chromosomes which un- 

 dergo crossing over, but which cannot twist about one 

 another with more than a certain degree of closeness. 

 The phenomenon of interference thus indicates another 

 point in which the chiasmatype hypothesis as developed by Morgan fits 

 the experimental facts. 



A further point is of interest in this connection. In Drosophila it is 

 only in the females that crossing over takes place; it is in the eggs, and 

 not in the spermatozoa, that new factor combinations appear as the result 

 of this process. The absence of crossing over in the male may be asso- 

 ciated with the fact that the F-chromosome carries no known factors; 1 

 the male is heterozygous for sex. In the fowl, in which the female 

 rather than the male is heterozygous for sex, it has been shown that 

 crossing over occurs in the male but not in the female. Crossing over for 

 ^ee, however, Castle (1921). 



FIG. 154. Dia- 

 gram illustrating 

 interference. Ex- 

 planation in text. 



