100 



AN INTRODUCTION TO MODERN GENETICS 



translocated fragment, the limitation on completeness being apparently 

 only the mechanical hindrance offered by the other chromosomes in 

 the closely packed nucleus. 



In the saUvary chromosome map the genes are more or less evenly 

 spaced along the chromosome, although even here there are regions in 



DEFICIENCY 



C/-nD 



o^ 



A B E F 



Salivary gland 



PACHYTENE 



INVERSION D ^E 



SEGMENTAL 

 INTERCHANGE 



Fig. A7. Pairing in Chromosomal Rearrangements. — In inversions, deficiencies, 

 translocations, etc., the pairing is betv/een homologous genes, and not between 

 the chromosomes as wholes. The figure shows diagrams of the pairing on the left, 

 examples from the salivary glands of Drosophila (after Painter) in the middle 

 and from pachytene figures on the right. The figure of a terminal deficiency (top 

 right) and segmental interchange (bottom right) were found in maize by McClin- 

 tock;^ the latter has been slightly simplified. The pachytene inversion (middle 

 right) is from Chorthippus (Darlington) and has also been slightly simplified. 



which the known genes are more concentrated. The chromomeres 

 (bands) however are quite evenly spaced and as each band probably 

 represents one or a few genes, it is probable that if all genes are known, 

 no regions would be more crowded than any other. The apparent 

 crowding of the known genes indicates regions in which the genes are 

 more mutable than normal, and the saHvary map therefore shows the 

 same sort of crowding as Muller's map in which the distance apart of 

 the genes represents the mutation frequency. 



The saUvary chromosomes reveal the presence of small inverted 

 duplications in the normal Drosophila melanogaster; for instance, in 

 1 Cf. Rhoades and McClintock 1935. 



