GENES, MOLECULES AND PROCESSES 



In cpistasy the expression of one gene difference conditions that 

 of another. The relationship is not reciprocal. With reciprocal 

 relations other Fg ratios are found. They are of three kinds according 

 to whether it is the two dominants, the two recessives, or one of 

 each which are needed for the combined effect. In any case only 

 two classes are recovered instead ot the three with epistasy. When 

 two dominants are needed we have complementary genes. The 

 classical case, indeed the first case, of any interaction to be described 

 and understood, is the production of coloured sweet peas from a 

 cross between two whites, the coloured-white ratio in Fg being 

 9 : 7. When two recessives are needed we have duplicate genes, in 

 a number of characters such as ligule — liguleless in wheat and oats. 

 The Fi may resemble both parents (when it is Ab X aB) but the 

 F2 gives a 15 : I segregation. 



The duplicate genes are so-called because the allelomorphs 

 appeared to be doing the same thing, whereas complementary 

 genes, being themselves dominant, were supposed to be doing 

 different things. In cereals the duplicate (or triplicate) genes are, no 

 doubt, doing the same thing since these plants are polyploid. But, 

 in diploid plants and animals, the 15:1 segregation may just as well 

 be taken to indicate a complementary action of recessives. This 

 interpretation is all the more plausible in view of the occurrence of 

 yet a third type, the complementary action of dominant and 

 recessive giving the 13 : 3 ratio which is shown by segregation for 

 ivory and yellow flower colour in suitable Antirrhinum crosses. 

 The recessive allelomorph of one gene is here called a suppressor of 

 the recessive allelomorph of the other. The term suppressor is again, 

 of course, merely an accidental tradition in this connexion. It might 

 just as well be applied to the complementary and duplicate relation- 

 ships, where it is the dominant allelomorph of one gene which, in 

 effect, suppresses the recessive of the other. Since dominance is 

 itself modifiable, the same kinds of interaction may underlie all 

 three. The difference may depend merely on which combinations 

 of dominants and recessives yield similar groups of phenotypes. 



Distinct from the five types of interaction discussed so far is that 

 where the Ab and aB combinations are not distinguishable. We 

 then have a 9 : 6 : i ratio and A and B are said to be additive. 

 Simple additive interactions arc rare, amongst major genes at least, 



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