86 SEX INHERITANCE 



lacticolor, but the lacticolors are females only. The 

 lacticolor grandmother has transmitted her pecuh- 

 arity visibly to half of her granddaughters, but to 

 none of her grandsons. 



In the reciprocal cross (Fig. 31) of lacticolor male 

 by grossulariata female, the daughters are like their 

 father (lacticolor), and the sons are like their mother 

 (grossulariata). This is so-called criss-cross inher- 

 itance. When the hybrids (Fi) are inbred, they give 

 lacticolor males and females and grossulariata males 

 and females in equal numbers. 



Sex linked inheritance, as shown by the foregoing 

 results, becomes intelligible if the factor for lacticolor 

 is carried by the chromosome Z. Its occurrence in 

 Z is indicated here by writing an L inside the circle 

 which represents that chromosome, while the allelo- 

 morphic character carried by the Z of the grossu- 

 lariata individual is indicated by writing G in the 

 circle. The W chromosome is indicated by the 

 blank circle. The two cases then work out as sho\\Ti 

 in the diagrams. 



The preceding analysis shows that the genetic 

 evidence calls for a mechanism in which the female is 

 heterozygous for- sex, since those of her eggs which 

 carry the factor for grossulariata all develop into 

 females, the others into males. In the case of 

 Abraxas there was for some years no positive cyto- 

 logical evidence in support of this view. Fortunately, 

 the cytological side is now in a much better position 

 owing to the work of Doncaster and Seller. 



Doncaster examined Abraxas cytologically, and 



