Chapter 21 

 INTERACTION OF GENES 



In Chapter 9 dihybrid ratios were described in which one 

 pair of genes affected one part of the plant and the other pair 

 affected an entirely different part. In the maize cross, PlPl 

 CrCr X plpl crcr, the genes PI and pi affect the color of the 

 plant, and Cr and cr determine whether the leaves will be smooth 

 or crinkly. Examples of dihybrid ratios where two different 

 regions of the organisms are involved could be cited by the 

 thousands. Less numerous but by no means rare are cases where 

 the two pairs of genes affect the same part of a plant or animal. 



In the simplest situation of gene interaction in a dihybrid 

 both pairs of genes affect the same part of the same organ. Four 

 different phenotypes are produced, for the AB, Ah, aB, and ab 

 classes are all phenotypically different from one another. In 

 the evening primrose, Oenothera Lamarckiana, the nonallelic 

 genes S and T^ together produce a yellow flower; S and v pro- 

 duce a flower that is old-gold in color; s and V together pro- 

 duce a sulfur-colored flower; and s and v interact to produce a 

 flower called '^gold-center." Thus a cross between a homozy- 

 gous sulfur-colored plant (mutant sulfurea) and a homozygous 

 old-gold (mutant vetaurea) would produce a yellow-flowered Fi. 

 The F2 would segregate into 9*SF (yellow) : 2>Sv (old-gold) : 3sF 

 (sulfur) : Isv (gold-center). This is a normal dihybrid ratio, 

 but all four genes are exerting an effect on the same part of the 

 organism. If one pair of genes is homozygous and the other 

 heterozygous, the offspring will show typical monohybrid F2 

 ratios, as: 



SSVv (yellow) X self = 3>SF (yellow) : ISv (old-gold) 

 ssVv (sulfur) X self = 3sF (sulfur) : Isv (gold-center) 

 SsVV (yellow) X self = 3<SF (yellow) : UV (sulfur) 

 Ssvv (old-gold) X self = ZSv (old-gold) : \sv (gold-center) 



Both pairs of genes alone will segregate 3 : 1 ratios in the F2, 

 but the actual color produced by the 5 and s genes will depend 



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