THE GROWTH OI- GENES 



For the Sex-Ratio super-gciic an inversion is indispensable. But 

 tliis by no means implies stability. The inverted sequence in which 

 it is found in D. pcrsiinilis is the normal sequence for pscudoohscura, 

 with regard to which the Sex-Ratio lies in an inverted sequence. 

 It has escaped from an inverted sequence into a normal one (pre- 

 sumably by double crossing-over); but to be preserved it has 

 required its normal allelomorph to be inverted. Indeed, in this 

 species as well as in D. azteca, Sex-Ratio has picked up three inver- 

 sions with which it is now combined — or rather locked, since escape 

 from multiple inversions is impossible (Fig. 84). 



These apparently random variations show us that the same 

 principle of chance which applies to the fixation of a sequence 

 floating in a sequentially inixed species, extends to the fixation of 

 a gene floating in a genically mixed sequence. They also show us 

 that Sex-Ratio is a super-gene but not always the same one. This 

 polymorphism, preserving the heterozygous condition and visible 

 both genetically and cytologically, enables us to see the changes 

 that Sex-Ratio has undergone, and that all super-genes can undergo, 

 in the course of a long evolutionary progress. 



The last and oldest stage in the submergence of a super-gene is 

 shown by a group of elements determining the cultivated character 

 of the two hexaploid cereals, wheat and oats. Li these, as in other 

 grain crops, the cultivated species have, by involuntary selection, 

 partly or wholly lost three properties that existed in their wild 

 ancestors : shattering of the ear, bearding, and toughness of the chaft. 

 Loss of a pair of chromosomes from one set exposes a group ot 

 these characters determined by the corresponding chromosomes of 

 a second set, and results in the appearance of a speltoid or fatuoid 

 mutant (Fig. 85), Tliese mutants, resembling the wild species 

 Triticum spelta and Avena fatua, are useless in cultivation. The 

 characters concerned are all related to the reproductive system of 

 the plant, yet they seem to have no inherent relationship in develop- 

 ment and they have therefore been assumed to be determined by 

 a group of genes. But this group never rccombines and has been 

 found by Nishiyama to be housed entirely in the short arm of the 

 chromosome that bears it. It is, in other words, a super-gene. 



The speltoid case recalls attention to the two opposed principles 

 concerned in building up genes. In the S allelomorphs there is an 



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