THE SEX MECHANISM 



integration of action within the cell to produce a single phenotypic 

 result. In speltoids and also in Sex-Ratio and in Lehistes there is 

 merely a combination of effects to produce several results that are 

 mutually compatible in whole individuals. The first arc complex 

 genes; the second are gene-complexes or super-genes. Wc have now 

 to see how, in systems we have already described, the two methods 

 of building are in fact combined. 



Fig. 85. — Spikelets of normal (N) and fatuoid (F) and various intermediate oats. 

 The fatuoids are distinguished by the simultaneous occurrence of an awn on every 

 flower of the spikelet and by the oval or sucker-mouth articulation surface sur- 

 rounded by dense pubescence. These characters are inherited as a unit and they 

 vary simultaneously, the heterozygote (H) and intermediate types, steriloid (ST) 

 and sub-fatuoid (SF), showing intermediate expressions of all the characters (after 

 Huskins, 1946). 



The Sex Mechanism 



Sex determination in most animals and plants, even in the haploid 

 liverworts, depends on the segregation of a difference at meiosis. 

 In the simplest cases this difference is indistinguishable from that 

 of the simplest gene. In mosquitoes and in the amphibia no recog- 

 nizable difference separates any pair of possible sex chromosomes. 

 But in mosquitoes partial sex linkage is found, so that there must 

 be an X — Y pair. How can we imagine a single gene setting off 

 the differentiation of males and females ? In maize the system has 

 been constructed experimentally. 



Maize has male and female inflorescences on different parts of 

 the same plant. Jones was able to make a dioecious plant out of it. 

 He began with a homozygous silkless stock, 5^ s^^, in which the 

 female inflorescence fails to develop. Introducing a second gene for 



339 



