GENETIC VARIATIONS 337 



man 48, in the fruit-fly 8, in the horse-worm 2, and so on. 

 This grouping of the genes of course has an effect on in- 

 heritance, since there is a linkage group for each chromo- 

 some pair. As we have seen, sometimes a chromosome is 

 broken into two, thus adding one to the number of linkage 

 groups. In other cases two chromosomes or parts of chro- 



Mi » (^ )) % y> 'i 



llT ^JK II ^H 



Figure 69. The chromosome groups (A to H) in various different 

 species related to Drosophila melanogaster (A), illustrating how that 

 of one species is producible from that of another by slight changes in 

 the union or separation of certain chromosome parts. After Metz 

 (1916). See text. 



mosomes may unite, decreasing the number of linkage 

 groups. 



When related species or varieties of animals or plants 

 are compared, it is often found that they have groups of 

 chromosomes so differing as to suggest that one has been 

 derived from another in the ways just described; that is, by 

 breaking, or by union, of certain chromosomes. This is for 

 example the case in different species of Drosophila and of 

 other genera related to Drosophila. Excellent figures show- 

 ing the chromosome groups in different species have been 

 published by Metz; some of these are copied In our fig- 

 ure 69. Type A, which occurs in the common fruit-fly, has 



