282 Criticmn of Hypothesis of Linkage and Crossing Over 



They interpret the normal Mendelian ratios as due to the presence 

 of the fiictors in different chromosomes. Th)is if two factors A and B 

 give the zygotic ratio of 9:3:3:1, they interpret this by locating 

 A and B in different chromosomes. It is obvious that if these factors 

 really are in different chromosomes, there can he no linkage and crossing 

 over, in Morgan's sense, so far as they are concerned. It appears, more- 

 over, from their statements, although the actual evidence is not directly 

 produced in sufficient amount to form a proof, that many factors arc 

 linked together in such a way that they behave during the segregation 

 period as an indivisible whole. For example, X, Y, Z may represent 

 characters which are generally completely correlated. Tested separately 

 by crossing with the corresponding recessive, x, y, z, we should find that 

 X:x\ 



Y:y\ ::3:1, but, and this is the vital point, generally, though not 

 Z:z] 



always, XYZ : xyz ::^ -.1. Such a behaviour is well described as 

 linkage, and would be undiscoverable if it were really absolute. Its 

 discovery only becomes possible when the linkage can be broken down. 

 The genesis of the hypothesis of crossing over is no doubt traceable to 

 the fact that the linkage is not absolute, except in rare cases. 



Perhaps the most essential element in the crossing over hypothesis 

 is the conception of the factors as being represented by the chromomeres 

 of the chromosomes, as if indeed they were numbered beads on a string, 

 each having a definite locus of its own. The factors are not only 

 restricted to a special chromosome, but to a special position in this. 

 So much is this the case that the authors have represented in the 

 Frontispiece of their book the position of no fewer than thirty-six 

 factors ; in one chromosome alone the positions of as many as nineteen 

 are shown. 



The authors take their standpoint (confessedly or by implication) on 

 the following grounds : 



(1 ) that the individuality of the chromosomes may be now accepted 

 as fully established, 



(2) that rod-like chromosomes actually occur in Droso-phila, 



(3) that in the prophases of meiosis in certain organisms whose 

 cytology has been more fully studied than that of DrosopJiila, the 

 homologous chromosomes may be observed in the positions recjuiied 

 by the hypothesis, 



