236 GENETICS IN RELATION TO AGRICULTURE 



suits. The Fi hybrids would then be of the genetic constitution AaBb- 

 CcDdEeFfGgHh. Such individuals produce gametes of the constitu- 

 tion abcdefgh only once in 256 times, so that when crossed back to tame 

 guinea-pigs which produce only gametes bearing the recessive factors, 

 0.4 per cent, of the males should be fertile. The percentage of fertile 

 males in successive generations of back crossing should then increase pro- 

 gressively as shown in the last column of Table XXXVII. As Detlefsen 

 himself, however, points out the close agreement of these calculated 

 figures with those actually observed is misleading as an indication of the 

 significance of the analysis, for it is doubtful whether simple segregation 

 of Mendelian factors provides an explanation of the entire phenomena. 

 It is rather strange in fact that only the males display this sterility, 

 and it is of interest to note, as Detlefsen points out, that several other 

 analogous instances of male sterility in animal species hybrids are known. 

 The yak, Bibos grunniens, crossed with the domestic cow, Bos taurus, 

 gives fertile female and sterile male offspring. Similarly the gayal, 

 Bibos frontalis, the gaur, Bibos gaurus, and the American bison, Bison 

 americanus, have been crossed with domestic cattle and have given fertile 

 female and sterile male hybrids. There is strong evidence that hybrids 

 of the banteng, Bibos sondaicus, and the zebu, Bos indicus, display similar 

 relations. When we consider the physiological relations between factors 

 and particularly the significant fact that probably no crossing-over 

 occurs in the males of this species, we feel inclined to attribute the male 

 sterility to other causes than to a mere sorting of factors having to 

 do with fertility. 



Partially Sterile Hybrids of Wheat and Rye. Thus far cases have 

 been considered in detail in which the species hybrids display a consid- 

 erable degree of fertility. At the other extreme stands a series of hybrids 

 which display sterility which is nearly but not quite complete. Such are 

 the hybrids between wheat and rye which Jesenko has subjected to 

 thorough experimental study. There can be no question that wheat and 

 rye are distinct species, in fact they have been universally assigned to 

 different genera. They seem to represent about the extreme limitations 

 of effective hybridization. Jesenko and others have been able to obtain 

 hybrids between wheat and rye only when wheat is used as the female 

 parent, consequently we are unable to compare the results of reciprocal 

 hybridization in this case. Even pollination of wheat with rye is suc- 

 cessful only about six times in one thousand as Jesenko found in over six 

 thousand trials with different species and varieties. The FI hybrids were 

 intermediate in general characters, although the relations of dominance 

 displayed in variety crosses was preserved in the species crosses. In 

 Fig. 100 is illustrated one of these hybrids and its two parents. The in- 

 creased size of the spike as compared with those of either parent is par- 



