234 AN INTRODUCTION TO MODERN GENETICS 



which is more or less confined to the Fungi, the sexual differentia- 

 tion is very sUght, and the four alternative complexes are very little 

 developed. 



In different types of sex determination, the alternative between the 

 A and G types of reaction is decided at different stages in the life cycle. 

 The fundamental reason for the decision, or rather its evolutionary 

 advantage, is that it ensures that fertilization shall be between gametes 

 from different animals. Probably, then, the original mechanism was an 

 alternative mode of reaction in the gamete itself. This survives in some 

 Algae, in which sex determination is performed by environmental 

 agencies working on the gamete. Usually, however, the time at which 

 the alternative is decided is pushed back in the life-cycle, probably on a 

 safety-first principle.^ Eventually, in the higher animals and plants, the 

 sex determination of a gamete has been pushed back to the fertilization 

 of the zygophase before. 



How far can one say that sex is the same thing in an organism with 

 determination of sex in the gamete and one where it is decided much 

 earlier? This is really the question of how far the A and G complexes 

 are the same in the two cases, since the A and G merely symbolize the 

 alternative modes of reaction which constitute the sex difference. 

 Hartmann seems to take maleness and femaleness as unanalysable 

 concepts, which we can naturally recognize as analogous in different 

 groups of organisms. This is a very abstract point of view. We can 

 perhaps get a more definite answer by considering an example. Sup- 

 pose that in a certain organism the sex of the gametes is determined 

 by whether they contain a substance a, which can stimulate the A 

 complex to develop and the cell to become a sperm; otherwise it be- 

 comes an egg. Suppose further that we are deaUng with a case of 

 phenotypic sex determination and that a comes from the external 

 environment. Now in the next step in evolution, perhaps some of the 

 organisms in the species succeed in producing a in their gamophase cells 

 before the actual gametes are formed; these organisms will then be 

 males and all their gametes will contain a and will develop as sperm. 

 But now the actual sex determination depends on whether the organism 

 forms a in its gamophase cells or not, and this alternative clearly will 

 depend on quite a different genetic basis to that which determined the 

 reaction of the gametes to the presence of a. Thus the sex determination 

 is now founded on a new alternative reaction system A'G' . The old A 

 and G must of course still be there to provide a basis for the effect of a, 

 ^ Cf. Haldane 1932a. 



