SUPPRESSION OF CROSSING-OVER 337 



in sex determinants and gives the two sexes in equal numbers. The 

 heterozygous condition is therefore essential for differentiation by 

 genetic segregation, but has no necessary connection with one sex 

 or the other. Whether the heterozygote is male, female or asexual 

 is indifferent to the origin and working of the chromosome 

 mechanism, which depends on the genetic reaction of the original 

 sex factors and the relative importance of the haploid and diploid 

 generation. It is therefore convenient in considering the mechanism 

 to speak of sex heterozygotes rather than of the heterozygous sex. 



As in the complex heterozygote, so in the sex heterozygote, the 

 differences (which determine sexual differentiation) behave as a 

 unit in inheritance. The sex heterozygote differs in that one of its 

 zygotic types can exist in a condition that is homozygous so far as 

 the sex differences are concerned and show free segregation of all 

 differences found in that condition. Permanence is secured by the 

 complementary functions of the sexes in reproduction. 



The two types of heterozygote have in common the two properties 

 of permanence and unity in inheritance. Clearly the permanence 

 could not be preserved without the unity, nor could differences 

 more complex than any that have been known to arise singly 

 have developed as a unit except through an accumulation of 

 changes such as requires permanence in the system accumulating 

 them. The one essential for these combined properties is, therefore, 

 a suppression of crossing-over between the differences that are 

 accumulated. Now crossing-over, as we have seen, is a normal 

 condition of metaphase pairing and segregation in all organisms. 

 Crossing-over is also a condition of preserving the similarity of 

 homologous chromosomes in species, without which divergence 

 sooner or later is bound to occur and pairing of any kind become 

 impossible. How then can a suppression of crossing-over be 

 brought about between chromosomes that continue to pair ? 

 Clearly there is only one means : a separation of the chromosomes 

 concerned into two parts, one part which is similar to a corre- 

 sponding part in the homologous chromosome and pairs with it, 

 another part which does not pair or cross over and contains the 

 differences that distinguish the two chromosomes. It is as we shall 

 see on the evolutionary changes in the relative sizes and positions 



