THE MENDELIAN PRINCIPLES OF HEREDITY 93 



linkage occurs : viz. that when factors lie in different 

 chromosomes they freely assort and give the Mendelian 

 expectation ; but when factors lie in the same chromo- 

 some, they may be said to be linked and they give 

 departures from the Mendelian ratios. The extent to 

 which they depart from expectation will vary with 

 different factors. I have suggested that the departures 

 may be interpreted as the distance between the factors 

 in question. 



A THEORY OF LINKAGE 



In order to understand more fully what is meant 

 by linkage on the interpretation that I have here 

 followed, it will be necessary to consider certain changes 

 that take place in synapsis. The sex chromosomes 



Ox g? ~tt± -K^ Ck 



<&* 



f 4* 



19 32 



g® <&=> 



5§f 



U, L M«w). Bn* 



Fig. 49. — Illustrating chiasma-type theory. 1 and 2, from Triton 

 cristatus, 3-46, chromosomes of Batracoseps attenuatus. Note especially 

 the chiasma shown in 13. (After Janssens). 



