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- 

 wasfo and Hance confirm these relations. 



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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 Rock 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 t offspring are bred 



