176 



PHYSICAL BASIS OF HEREDITY 



black pigment in the wings. When a dark (grossulariata) 

 male is mated to a light (lacticolor) female, both sons and 

 daughters are dark (Fig. 74). If these are inbred all the 

 F2 sons are dark, half the daughters are dark, half light. 

 As the diagram shows, the distribution of the Z-chromo- 

 some furnishes the mechanism by means of which we can 



GR05SUU\R!ATA 9 OG 



UCTICOLOR 



O ® GEFt!1 CELLS ® 



LACTICOLOR 9 OL 



\ GROSSULARIATA c5 GL 



,0 ©^GERM CELLS ^,.(g) (l)^ 



O ®'^ o © 



© ® 



GROSSULARIATA 9 OG LACTICOLOR 9 OL GROSSULARIATA d LG i^CTlCOLOR 6 LL 



m 



Fia. 75. — Cross between Abraxas grossulariata female and lacticolor male. 



explain, as in Di'osophila, the process of sex-linked inheri- 

 tance in this moth. 



The reciprocal cross is shown in the next diagram (Fig. 

 75) in which a dark (gross.) female is mated to a light 

 (lact.) male. The daughters are light like the father, the 

 sons dark like the mother — criss-cross inheritance. The 

 daughters get their one Z-chromosome carrying the light 



