SEX-CHROMOSOMES AND INHERITANCE 175 



If the sex-chromosome that carries the sex-linked genes 

 in birds and moths be symbolized by Z, and its homologue 

 that occurs in the female by W, the scheme for sex-deter- 

 mination is that shown in Fig. 73 : The eggs of the female 

 extrude either one or the othe^ sex-chromosome. If Z 

 stays in, and this egg is fertilized by a sperm (Z-bearing 



LACTICOLOR 9 OL 



GROSSULARIATA 6 CG 



O © G™ CEILS ® 



GR055ULARIATA Q OG , 



GROSaiLARlATA d LG 



O ® GERM CELLS,. ® ® 



o ®- 



LACTICOLOR <? OL 



O ®' ® ® 



GR0SSULAR1ATA9 OG CR0S5ULAR1ATA c5 GL 



® ® 



GR055ULAR1ATA 6 GC 



Fig. 74. — Cross between Abraxas lacticolor female and groasulariata male. 



also) a male (ZZ) is produced; if W stays in, and the egg 

 is fertilized by a ^-bearing sperm, a female (WZ) is 

 produced. The way in which sex-linked characters are 

 transmitted may be illustrated by the inheritance of a 

 color difference in the currant moth Abraxas. The wild 

 species (grossulariata) has a mutational variety called 

 lacticolor, that differs from the former by having less 



