208 



THE THEORY OF THE GENE 



According to Stevens, in the chick the male appears to 

 have two large chromosomes equal in size (Fig. 119), 

 presumably X 's ; the female has only one of these. Shi- 

 wa2:o and Hance confirm these relations. 



GROSSIMRIATA 9 .01 



LACTICOLOR d II 



O ® GERM CELLS ® 

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LACTICOLOR 9 01 



GR055UL^R1ATA d LI 



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a ®""' o ©"' ^® ©• "® ^® 



GR05SULAR1ATA 9 OL LACTICOLOR 9 01 GR055ULARIATA d 1 L U\CT1C0L0R d 11 



Fig. 121. 

 Sex-linked inheritance in the currant moth, Abraxas. 



The genetic evidence for birds is beyond dispute. It 

 comes from sex-linked inheritance. If a Black Langshan 

 male is mated to a Barred Plymouth Eock female, the 

 sons are barred, the daughters are black (Fig. 120). This 

 is expected if the differential genes are carried by the 

 Z-chromosomes, because the daughter gets her single Z- 

 chromosome from her father. If the F^ offspring are bred 



