BREEDING SYSTEMS 



certain recombinations amongst the general genes had broken down 

 the action of the switch gene. They had in fact prevented the XY 

 combination from inhibiting tlic development of the ovary. They 

 had thus turned males back into hermaphrodites, the condition from 

 which maleness and fenialeness must have recently arisen in both the 

 parent plants. 



In animals with long-established dioecy, the same breakdown 

 occurs in hybrids, but it produces sterility. Race crosses in the moth 

 Lymantria dispar, for example, give intersexes. In these, female 

 development is superseded by male, or vice versa, in the course of 

 growth, the time of the change determining the grade of the 

 intersexuality, as we saw in Chapter 7. 



In Drosopliila we can go two steps further. Intersexes can be 

 produced by altering the numbers of chromosomes present. As we 

 have already seen, an extra set of autosomes with the ordinary 

 two X's of the female turns it into an intersex. The presence or 

 absence of Y makes no difference. Evidently, therefore, the switch 

 gene effect of X-Y segregation depends on the differences in pro- 

 portion of autosomes and X chromosomes in XX and XY flies 

 (Fig. 55). Again, as in Petunia and Nicotiana, genes whose differences 

 are not segregating are making the switch mechanism work. 



The second step in Drosopliila is in shov/ing that the switch gene 

 itself is compound. When the X chromosome is broken by X-rays, 

 and its fragments are lost, its effect is diminished: females are 

 turned into intersexes. The X chromosome, or at least its 

 differential segment, is thus a super-gene in respect of sex deter- 

 mination, although an aggregate of genes in other respects. 



In genetic terms, breeding systems are now seen to be of two types. 

 First, all those giving inbreeding and some giving outbreeding, like 

 protandry and cyclical hermaphroditism, are controlled by general 

 gene systems which do not show any special variation within the 

 species. Their operation does not depend on diversity, that is on 

 segregation. Secondly, the chief cross-breeding systems, sex and 

 incompatibility, on the other hand, require segregation and therefore 

 depend on the action of switch genes. But the existence of the 

 mechanism which these genes switch, depends on a general gene 

 system which does not need to show segregation. 



On this basis new systems can come into existence by adjustment, 



258 



