UKl.LDlNCi SVSTi;MS 



The controlling incompatibility gene, it will be noticed, takes us 

 a new step in genetical inference. We recognize its allelomorphs as 

 distinct, not by a difference in behaviour of the various genetical 

 kinds of pollen in styles carrying the same allelomorphs, for these 

 are alike in that none will grow; but by the difference between such 

 behaviour, on the one hand, and the growth of pollen carrying any 

 allelomorph in a style carrying different allelomorphs, on the other. 

 It is just the same as the way in which we sec the effects of inversion 



AB Ab aB ab 



AB 



Ab 



aB — 



ab 



Fig. 6o. — Hctcrothally determined by two loci in fungi. Successful fusion, indicated 

 by +, occurs only where the two hyphae ditl:er at both loci. All other combinations, 

 whether alike at both or only one of the loci, fail to achieve fusion, indicated by — . 



or interchange at meiosis; not in homozygotes, which show normal 

 behaviour, but in the structural heterozygotes. 



With suitable breeding experiments the linkage of this gene, S as 

 it is called, with other genes can be established and its mutations 

 recorded. Of course in a fully operative system the number of 

 allelomorphs of the S gene can never be less than three and is always 

 in fact much more numerous, hi the small species Oenothera 

 organciisis, with a total wild population of perhaps no more than 

 500 plants, 35 allelomorphs have been identified. In the large species 

 of Red Clover, Tiifoliuiii pratensc, one group o^ 24 plants had 41 

 dift'crcnt allelomorphs and another group ol 20 j->lants luid 37 



246 



