94 



AN INTRODUCTION TO MODERN GENETICS 



lO 



20 



TEMPERATURE 



30 



B 



Fig. 42. Variation of Crossing-over. — A In chromosome III with age of female 

 B in chromosome II with temperature,^ in Drosophila melanogaster. 



(After Bridges, and Plough.) 



to reconcile this with the peculiar method of pairing of the autosomes 

 in the male, and it is probable that the crossing-over is of an abnormal 

 type and occurs at mitosis some time before meiosis (p. 373).^ 



4. The Physical Reality of the Cross-over Map 



The demonstration of the physical reality of the cross-over map 

 came from the discovery of cytologically observable alterations in the 

 chromosomes which could be correlated with alterations in the position 

 of definite sections of the cross-over map. 



TtRMINAL OeriClCNCY 



INTEWTITIAL OEFICIKNCT (OELCTIOn) 



. Q O 



o, bcdefghi -> 0, bed {efghi lost) 

 o, bcdefghi -^ a, bghi {cdef lost) 



.SL 



c, bcdefghi — > a, bgfedchi 



TRANSLOCATION 

 — O" 



^ 



SEGMENTAL INTERCHANGE 



X. 



n 



a, bcdefghi 

 m, nopqrs 



a, bcdefghi 

 m, nopqrs 



0, bcopqdefghi 

 m, nrs 



a, bcdpqrs 

 m, noefghi 



Fig. 43. Chromosome Rearrangements. — On the left are shown the chromo- 

 somes before breakage, in the middle after breal<age and rejoining. On the right 

 the nature of the rearrangement is indicated by changes in the sequence of letters, 

 which represent the genes (the centromere is symbolized by a comma). It is 

 doubtful whether terminal deficiencies ever actually occur (cf. p. 368). 



* This oirve requires some correction, cf. Smith 1936. ^ Whittinghill 1937. 



