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sterile. In other words, in every 1', plant one-hiilf the game- 

 tophytes were sterile. It is easy to see why XO and OY arc 

 fertile, also why 00 is sterile (lacking both factors); but why 

 should XY be sterile when it has both factors ? Belling explains 

 it by saying that ganietophytes are unHkc sporophytes in that 

 they normally have single factors instead of double factors. The 

 germinal capacity of a gametophyte is just one-half that of a 

 sporophyte. It is as if a gametophyte were "supersaturated" 

 by a double factor. Such a 

 situation is abnormal for a 

 gametophyte and brings ab- 

 normal results. Therefore 

 the gametophyte having the 

 abnormal double dose {XY) 

 is just as sterile as the 

 gametophyte with no dose 

 {00).^ 



In developing the F2 

 ratios of course only the fer- 

 tile gametophytes function. 

 XY and 00 are eliminated, 

 so far as posterity is con- 

 cerned, so that we have to 

 deal only with the chance 

 matings among the fertile 

 gametes {XO and OY). According to the laws of chance there 

 are four possible matings between these gametes (fig. 22). Out 

 of the four resulting F2 sporophytes two would evidently pro- 

 duce only fertile gametophytes and would remain fertile as long 



^ Belling's rather awkward assumption to the efTect that the 

 gametes with the double dose {XY) are non-functional on account of 

 being "supersaturated" might be improved upon by the following. 

 Assume race A has complementary factors A' and O for fertility, while 

 race B has a similar set of complementary factors, () and Y, located on 

 the corresponding chromosomes. The scheme then works out as before, 

 gametes of the BO and AT' formulas both being non-functional for the 

 same reason that a necessary pair of comiilemcntary factors is not 

 present. 







® 



Fig. 22. — Diagram showing how 

 the F2 would be produced according 

 to Belling's idea of semi-sterility. 



