Chromosomes and Linkage Groups. 77 



XIII. CHROMOSOMES AND LINKAGE 

 GROUPS COMPARED. 



On the basis of its chromosomes, Drosophila virilis would be expected 

 to possess six groups of linked genes. Five of these, including the 

 sex-linked group, should be of approximately the same size as regards 

 amount of crossing-over, and the sixth should be relatively very 

 small, both in respect to amount of crossing-over and to number of 

 genes. Six groups have been identified, as indicated in the above 

 account. Of these the sex-hnked group (group I) includes far more 

 genes than any of the others, as would be anticipated from the fact 

 that sex-linked mutations are more readily detected than autosomal 

 ones. The autosomal Unkage groups (II-VI) contain respectively 

 4, 6, 3, 5, and 2 characters, and do not, as yet, show the expected 

 difference distinguishing one group from the other four. As to 

 amount of crossing-over, it is too early to compare them, except in 

 a general way. Groups II, III, and V show sufficient crossing-over 

 to indicate that they will probably equal the sex-hnked group in 

 map-length when more characters are obtained. On this basis, 

 these four groups (I, II, III, and V) are identified with four of the 

 large chromosomes. Groups IV and VI, however, although con- 

 taining 3 and 2 characters, respectively, give no crossing-over, 

 and the question arises as to which is to be identified with the re- 

 maining large chromosome and which with the small ???-chroniosome. 

 This question can not be answered with certainty at present, but 

 the following three lines of e\'idence favor the view that group 

 VI represents the ?72-chromosome : (1) One of its two characters 

 appears to be a deficiency for the other, indicating that a small 

 ''region" is involved. (2) Both characters resemble characters 

 in group IV (representing the 7?i-chromosome) of D. inelanogaster 

 (see above under group VI). (3) The absence of crossing-over 

 in group IV of virilis may possibly be due to the fact that the char- 

 acters involved have arisen in stocks obtained from different localities 

 (see above under group IV). Possibly the "fourth" chromosomes 

 in these stocks are sufficiently different to inhibit crossing-over. 

 An attempt is being made to test this view. Another explanation is 

 that group IV represents a long chromosome, and that crossing- 

 over is infrequent in this chromosome because of the presence 

 of one or more genes reducing crossing-over, analogous to those known 

 to have this effect in certain races of D. inelanogaster (Muller, 1916; 

 Sturtevant, 1919; Detlefsen, 1920; Gowen and Gowen, 1922). 



In the other species of Drosophila, exclusive of inelanogaster and 

 simulans, Uttle has been published regarding the autosomal linkage 

 groups. It may be stated, however, that in both willistoni and 

 obscura the number of such groups agrees with or at least does not 



