242 Mechanism of Mendelian Heredity 



of the cytologists and afterward separate again. It 

 had been observed by Janssens that in this stage of 

 fusion and subsequent separation a partial twisting 

 and a partial exchange between two chromosomes may 

 take place. Morgan assumes that this exchange 

 accounts for certain deviations in the ratio of link- 



A 



B 



a A_ 



I I 



FIG. 40 



FIG. 41 



FIG. 42 



age. If in Fig. 40 the white and black signify two 

 homologous chromosomes I and I x containing the 

 two pairs of homologous factors AB and ab respectively, 

 the synapsis state would be as in Fig. 41. If the 

 separation were complete, either I or its homologue I x 

 might be lost in the maturation division of the egg. 

 If, however, the synapsis is slightly irregular, as in 

 Fig. 42, where the chromosomes are slightly twisted, 

 I and I, will not separate completely but an exchange 



