236 GENETICS IN RELATION TO AGRICULTURE 
sults. The Ff, hybrids would then be of the genetic constitution AaBb- 
CcDdEeF{GgHh. 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 XX XVII. 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 conrpare 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 Fy 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- 
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