THE INTERACTION OF GENES: THE EFFECTS l6l 



purple eyed colour (Ilnd Chr) which is suppressed by a factor su lying 

 in the Ilird chr. The compound pr pr su su has normal eye colour. A 

 suppressor is not the suppressor of a locus but of a gene. Thus the com-"^ 

 pound pr pr su su is not phenotypically the same as a deficiency of the 

 pr locus : it is the same as -\-^^ ^-^^ Similarly if A is epistatic to B,AABB 

 is not the same as AA dcf.B, but the epistatic relation means that there 

 is no difference between AABB, AAbb, or AAbb. 



Fig. 77. Modification of the Dihybrid Ratios. — On selfing an individual which 

 is heterozygous for the two unhnked factors A and B, one obtains 9 zygotes con- 

 taining both A and 6, 3 with A and b, 3 with 6 and o, and 1 with neither A nor 6. 

 If both A and B are completely dominant, the phenotypic ratio will be modified 

 from the fundamental genotypic ratio of 9 yAB : 3 Ab : 3o B : 1 ah according as the 

 classes are separately distinguishable. Some of the possible modifications are as 

 follows: 



9:3:3:1. Factors simple dominants, e.g. A = rose comb, B = pea comb, 

 AB = walnut comb, ab = single comb in fowls. 

 12:3:1. A epistatic to B, so that AB looks the same as A; e.g. coat colours 



in mice A = AB = grey, B = black, ab = chocolate. 

 9:3:4. B ineffective in absence of A (or a epistatic to B); e.g. mice A == basic 

 factor for colour formation, a = albino, B = black. 

 9:7. A and B complementary; e.g. A and B are the two factors (known as 

 C and R) necessary for colour formation in sweet-peas. 

 13 : 3 : 1. 6 is a suppressor of A, having no effect itself; e.g. A is the factor 

 for colour in fowls, B its suppressor (dominant white). 

 9:6:1. A and B are polymeric factors, each producing the same effect, the 

 two effects being added in AB; e.g. polymeric factors for colour 

 of the grain In wheat. 



4. Modifying factors 



Factors like the suppressors mentioned above, which have no effect 

 unless they occur in compounds with some other definite genes, are 

 known as modifiers or modifying genes for the factors with which they 

 are effective.^ Suppression is an extreme case of modification; usually 

 the effect is less marked. Many modifying genes probably do not 

 differ in any fundamental way from ordinary genes: they merely 

 produce a slight effect which is concealed by the "factor of safety" 

 (p. 185) characteristic of the dominant wild-type genes but which can 

 be revealed in the less equihbrated mutant forms. Thus scute in 

 D. melanogaster has a sub-threshold effect in certain bristles which is 

 not revealed until the genotype is "sensitized" by the introduction of 

 another bristle-gene, in this instance Hairless^ (p. 371); scute might 

 therefore be referred to as a modifier of Hairless. In other cases, where 

 a particular allelomorph introduces a completely new reaction into the 



^ Bridges 19 19. - Sturtevant and Schultz 193 1. 



