ADJUSTMENT AND BALANCE 



Scgregational sterility, on the other hand, results where the con- 

 tributions of the two parents are relatively balanced as wholes, but 

 balanced in different ways, so that segregation and recombination 

 of their parts give unbalance. This unbalance expresses itself gcno- 

 typically in plants in the haploid generation, in the gametes that 

 will give the Fo or backcross; but in animals where, as we have 

 seen, unbalanced gametes regularly survive and function, scgre- 

 gational sterility appears only as the death of the zygotes of the Fo 

 or backcross. 



When the unbalance due to segregation is not so drastic as to lead 

 to death, it may still cause a developmental upset in the individuals 

 obtained in a backcross or Fg. This upset may, of course, express 

 itself at germ cell formation so that it leads to a reduction in 

 fertility. In this way segregation in the F^ may lead to genotypic 

 sterility in the next generation (Fig. 77). These relationships are 

 well shown by the comparison of the Fj and the backcross 

 generations in the cross between the two grasses Lolitim pcrennc 

 and Fcstuca pratcnsis examined by Peto (Table 20 and Fig. 56). 



Reciprocal crosses were made. The parents were different indi- 

 viduals with slightly different chiasma frequencies but they were all 

 highly fertile except one, which was itself a varietal hybrid and 

 doubtless owed its bad pollen to segregation. Both of the reciprocal 

 Fj's had chiasma frequencies similar to the parents, and showed no 

 evidence of structural hybridity, apart from 2 per cent of potential 

 bivalents unpaired in two plants. There was, however, a great 

 reduction in pollen fertility, again presumably due to segregation. 



The results of this segregation appeared clearly, though in a dif- 

 ferent way, in the next generation, which was the first backcross 

 to Lolimn pcretine. Here 5 plants had normal chiasma frequencies, 

 no failure of pairing and high fertility; but 2 plants had chiasma 

 frequencies greatly reduced, in fact below the level necessary for 

 regular pairing. Their fertility was therefore greatly reduced. But 

 this reduction in fertility, in contrast to that of Fi, was presumably 

 due in a great part to the failure of pairing. Now the dissimilarity 

 of the pairing chromosomes must have been at its greatest in Fj . 

 The decrease of chiasmata and pairing cannot therefore itself have 

 been due to an increase in dissimilarity. Rather the failure of pairing, 

 and with it the sterility, must have arisen from a genie upset 



234 



