SEX DETERMINATION 235 



SO that the species has now actually got an alternative reaction system 

 ^ + ^' and G + G'. 



This example has been chosen so that the evolutionary change takes 

 place entirely within the gamophase. The change from gamophase to 

 zygophase sexuality will also involve a difference in or addition to the 

 alternative complexes, but there is no obvious reason why it should be 

 a difference of another kind to the differences which cause the evolu- 

 tionary pushing back of sex determination into the gamophase itself. 



In some organisms, when the sexual differentiation is slight, we may 

 be tempted to neglect the alternative system altogether and think of 

 the sex determination as entirely controlled by the realizers. For 

 instance, in tetrapolar sexuality Mainz has suggested that it is unneces- 

 sary to invoke more than the A and B factors. But, since we believe 

 that any differentiation always involves many factors, if not the whole 

 genotype, we must even here admit that the A and B factors can only 

 be active in co-operation with other genes, and in so far as the A and 

 B factors have different effects, the other genes must be regarded as 

 some sort of an alternative reaction system, only one in which the 

 alternatives are not very highly developed. 



3. The Differential Sex Genes or Realizers 



The other half of the sex determining mechanism are the differential 

 sex genes or "reaHzers." 



The complexity and efficiency of the differential sex genes varies very 

 much in different groups. In the fish, as we have seen, there is a highly 

 complex alternative reaction system controlled probably by a single 

 gene or even by external conditions alone. In most cases the differential 

 is more complicated. In the first place, it may involve both male and 

 female factors, which build up the male and female substances which 

 are the immediate stimulants of the alternative reactions. Secondly, 

 either or both of the male and female differentials may contain many 

 factors. Since one sex must always be heterozygous for one of these sets 

 of factors, a differential system containing several genes can only be 

 stable if crossing-over is suppressed between them. It is a very general 

 rule, therefore, that crossing-over is reduced in the heterogametic sex, 

 and this suppression is often, as Haldane has pointed out, not confined 

 to the section of the chromosome where the differential factors He but 

 aflfeas all the chromosomes. Crossing-over within the actual differential 

 set may, however, be more efficiently suppressed by collecting the 

 heterozygous genes together into a certain part of the X chromosome 



