Mechanism of Mendelian Heredity 243 



will take place, part of Ii and I becoming exchanged. 

 This would result in the formation of two mixed chro- 

 mosomes Ab and aB (Fig. 42). This partial ex- 

 change of homologous chromosomes, which Morgan 

 calls "crossing over,'* occurs, as he found in Drosophila, 

 in the egg only, not in the maturation division of the 

 sperm. He informs me that in the silkworm moth 

 Tanaka fotmd that it occurs only in the male, while in 

 Primula it takes place both in the ovules and in the 

 pollen as shown by Gregory. 



Morgan and his fellow-workers have put this theory 

 to numerous tests by breeding experiments and the 

 results have fully supported it. According to the 

 chromosome theory linkage should occur only when 

 factors lie in the same chromosome. Hence it should be 

 possible, on the basis of this linkage theory, to foretell 

 how many linkage groups there may occur in a species; 

 namely, as many as there are chromosomes. In 

 Drosophila there are four pairs of chromosomes, and 

 Morgan and his fellow- workers found only four groups 

 of linked characters.* This agreement can be no mere 

 accident. 



Carrying the assumption still farther, these authors 

 were able to show that each individual character has 

 in all probability a definite location in the chromosome, 

 so that it seems as if each individual chromosome 



' The number of hereditary characters examined to test the theory was 

 over 130. 



