Mechanism of Mendelian Heredity 241 



in definite cases and these cases he interpreted as being 

 due to "gametic coupling." 



These phenomena demonstrate the existence of a complex 

 interrelation between the factorial units. This interrela- 

 tion is such that certain combinations between factors may 

 be more frequent than others. The circumstances in which 

 this interrelation is developed and takes effect we cannot 

 as yet distinguish, still less can we offer with confidence 

 any positive conception as to the mode in which it is exerted. 



Morgan has given an ingenious explanation of these 

 deviations on the basis of the chromosome theory of 

 Mendelian heredity. He assumes that they occur in 

 those cases where the two or more characters are con- 

 tained in the same chromosome. In that case the two 

 factors lying in the same chromosome should generally 

 be found together. Such was the case for instance in 

 the experiments with flies having red eyes and yellow 

 body colour versus white eyes and grey body colour, the 

 character for white eyes and yellow body being located 

 in the X chromosome (see preceding chapter), or in the 

 experiments on Abraxas. These phenomena are called 

 linkage, and the numerical results of linkage were given 

 in the preceding chapter in connection with the crossing 

 of sex-linked characters. 



We have already mentioned that before the matura- 

 tion division occurs the homologous maternal and 

 paternal chromosomes fuse — the so-called synapsis 



* Bateson, W., loc. cil., p. 157. 

 16 



