266 



AN INTRODUCTION TO MODERN GENETICS 



by observations of loop pairing at zygotene in the hybrids, or of the 

 formation of chromosome bridges at anaphase following crossing over.^ 

 In Drosophila pseudoohscura^ different local races within the species have 



X 



S M 



Fig. 118. Comparison of Chromosomes in Drosophila melanogaster and 

 simulans. — A shows the comparative crossing over maps of X, 2nd and 3rd 

 chromosomes (S = simulans, M= melanogaster); notice the large inversion in 

 the right arm of the third chromosome. The spindle attachments are marked with 

 arrows, 6 shows the pairing of the inverted region in salivary glands of a hybrid. 

 C shows the left end of the X chromosome in a hybrid; structural differences can 

 be seen at the points indicated by the arrows. 



(Ater Patau.) 



different gene arrangements in their chromosomes. These can best be 



investigated by studies on the pairing in the salivary glands of hybrids. 



The differences found are mostly inversions, and are sufficiently 



numerous and compHcated to enable one to draw up a phylogenetic 



tree based on the way in which one arrangement might be transferred 



into another. Dobzhansky illustrates the method of argument by the 



following example : suppose we have three gene arrangements abcdefghiy 



^ Darlington 1936^ Upcott 1937. 



2 Sturtevant and Dobzhansky 19363 Keller 1936, Rev. Dobzhansky 1937^. 



