208 GENETICS IN RELATION TO AGRICULTURE 



homozygous for grossulariata, therefore (LZ)(LZ), and half heterozygous 

 (LZ)(IZ). Of the females half are grossulariata W(LZ) and half lacticolor 

 W(IZ). No laciicolor males are produced in this generation, but they 

 may be obtained from matings of heterozygous grossulariata males (Z/Z) 

 (IZ) with lacticolor females W(IZ}. The reciprocal cross requires no 

 special explanation, since it is perfectly clear from the diagram just how 

 the lacticolor factor is transmitted in such cases. Throughout, the whole 

 set of experimental evidence duplicates exactly the relations found to 

 exist for the inheritance of white eye color in Drosophila except that the 

 sex relations are reversed. 



The cytological relations in Abraxas do not appear to rest upon as 

 firm a basis as those in Drosophila. Apparently there are normally 56 

 chromosomes in both the male and female, and no pair are obviously 

 unequal in either sex. Apparently then the JF-chromosome in the female 

 is about the same size as the homologous Z-chromosome, but like the 

 Y in Drosophila it is a neutral chromosome, i.e., it carries none of the 

 dominant sex-linked factors. 



Some additional cytological evidence is provided by examination 

 of lines giving aberrant sex ratios. Doncaster discovered certain strains 

 in which some of the females gave only female offspring, others only a 

 few sons, and still others the normal 1 : 1 ratio. In these strains the 

 males had 56 chromosomes, but the females only 55. As Bridges points 

 out, if 56 is the normal chromosome number for the females of Abraxas, 

 then those females having 55 chromosomes may be regarded as of the 

 ZO type, corresponding to the XO males in non-disjunctional strains of 

 Drosophila. Such females produce eggs some with 27 and others with 

 28 chromosomes. If as Doncaster's early observations seemed to show, 

 the odd chromosome ordinarily is included in the polar body, then the 

 eggs would contain mostly 27 chromosomes, and these on fertilization 

 would give 55 chromosome zygotes, presumably females of the ZO type. 

 Later observations of Doncaster's, however, do not confirm the conclusion 

 that 27 chromosome eggs are more frequent than those containing 28 

 chromosomes. Moreover, although this is perhaps not a very weighty 

 argument, it is not clear why ZO females in Abraxas, if such exist, should 

 not be sterile like their counterparts, the XO males in Drosophila. 



It is of considerable interest that exceptions in the transmission of the 

 sex-linked character lacticolor occur in Abraxas just as they do in Dro- 

 sophila. The mating grossulariata female by lacticolor male should give 

 only lacticolor females and grossulariata males. However, Doncaster 

 found among 611 females, the offspring of 27 such matings, three gros- 

 sulariata females and two of these were in the same brood. Assuming 

 that the two which were in one brood represented cases of secondary 

 non-disjunction, it would appear that primary non-disjunction in Abraxas 



