SEX-CHKOMOSOMES AND INHERITANCE 167 



the way in which he ihlierits the factors carried by 

 the other chromosomes (autosomes), because X and Y 

 differ from each other in a wav in which no other chromo- 

 somes differ. 



The recessive gene for white eyes (w) in Drosophila is 

 carried by the X-chromosome. It is inherited in the fol- 

 lowing way (Fig. 69) : When a male with white eyes (w) 

 is mated to a red-eyed female (TFTF), the F\ sons and 

 daughters have red eyes. When these are bred to each 

 other, all the daughters have red eyes (50 per cent.), half 

 the sons have red eyes (25 per cent.) and half the sons 

 have white eyes (25 per cent.). The ratio, irrespective 

 of sex, is three red to one white, but the white-eyed flies 

 are found only amongst the males. In the diagram (Fig. 

 69), the relation of these results to the sex-chromosomes 

 is shown. The X-chromosome that carries the normal 

 gene (wild type) which gives red eyes is indicated by 

 W. The X-chromosome that carries the gene for white 

 eyes is indicated by w. The rod with a bent end stands 

 for the Z-chromosome. 



The Fi daughters contain one of each kind of X-chro- 

 mosome. The Fj sons only one kind. The recom- 

 binations that give the F2 results are shown in the middle 

 of the lower part of the diagram. Half of the females 

 are seen to be homozygous for the wild-type gene (W). 

 They should never transmit white eyes, and they do not. 

 The other half of the females are heterozygous (Wtv), and 

 if mated to a white-eyed male should give 50 per cent, 

 red-eyed males and females, and 50 per cent, white-eyed 

 males and females. This they do. The red F2 sons (W) 

 should never transmit white eyes, nor the white-eyed sons 

 (w) transmit red eyes. These relations are also known 

 to hold. 



The reciprocal cross (Fig. 70), viz., a white-eyed 

 female (luw) to a red-eyed male (W) gives red-eyed 

 daughters (wW) and white-eyed sons (iv). If these F^^s 



