The Bearing of Cytological Research on Heredity. 345 



apparently bizarre character if we assume that the production of each sex- 

 linked character depends upon something (which we may call a " factor " or a 

 " gen ") that is borne by the X-chromosome. I have already emphasised the 

 fact that the sons derive this chromosome from their mother. In the cross 

 just considered the sons therefore inherit with this chromosome the white 

 eyes of their mother ; the daughters, on the other hand, are red-eyed like their 

 normal father, because they receive from him in every case a normal 

 X-chromosome bearing the factor for the dominant red colour (fig. 3, A). 



A 



B 



Diploid Nuclei XX 



XX 



Fig. 3. — Diagram of the relations of the sex-chromosomes to sex-linked hei'edity. Any 

 normal (dominant) sex-linked character {e.g., red eyes) is assumed to depend on the 

 presence of a particular factor contained in the X-chromosome. Loss or modification 

 of this factor produces a corresponding recessive (e.g., white eyes). X now 

 becoming x. 



A. Criss-cross heredity, when the double recessive, white-eyed female (xx) is 

 paired with the normal, red-eyed male (XY). The heterozygous daughters (zX) 

 are red-eyed, with white-eye recessive ; the sons (zygotes xY) white-eyed. 



B. History of the following generation, produced by crossing ,rX and xY. The 

 offspring of both sexes are now indifferently white-eyed (xx, xY), or red-eyed 

 (Xr, XY). 



This has been tested in many ways, with results always in accordance with 

 the hypothesis. Very convincing evidence in its favour has recently been 

 obtained by Bridges through the study of a particular race of Drosophila, 

 which regularly shows about 10 per cent, of exceptions to the "criss-cross" 

 type of heredity. This holds true for all the sex-linked characters thus far 

 tested. Bridges' analysis — too intricate to be entered upon here — led to the 

 conclusion that these exceptions are due to a failure of the two X-chromosomes 

 to undergo disjunction in the reduction division of the female, so that the 

 mature egg sometimes receives both X-chromosomes, sometimes neither. Eggs 

 of the XX type might be expected to produce females even if fertilised by 

 the no-X type of sperm, the XX combination (characteristic of the female) 



