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THE AMEBIC AN NATURALIST 



[Vol. XLIX 



Again, if such a change occurs that A becomes A' (Fig. 

 3), a series of triple allelomorphs giving monohybrid re- 

 sults with each other, is formed. "A" is allelomorphic 



But there are other character determinants in the first 

 pair of chromosomes. What happens if both "A" and 

 "B" become changed? There are two possibilities, as 

 shown in the two parts of Fig. 4. If one of the members 

 of the pair of homologous chromosomes becomes abC 

 while the other remains ABC, there is a positive corre- 

 lation between the inheritance of "A" and "B." On the 

 other hand, if the change is such that the two chromo- 

 somes are aBC and AbC, there is a negative correlation 

 between A and B. In other words, the determinants re- 

 main correlated in the same way they entered the com- 

 bination. There may be breaks in these correlations, 

 however, as Morgan has shown in Drosophila; and these 

 breaks in correlation occur in a constant ratio. Diagram- 

 matically, it may be said that A and B are always the same 

 distance apart in the chromosome structure and that the 

 determinants " cross over" from one member of a pair 

 to the other every so often. All of the gametes in the 

 first case are not ABC and abC, for example. Some of 

 them will be AbC and aBC. And the same percentages 

 of these cross overs are found in the second case where 

 "A" and "B" are correlated negatively. Furthermore, 



