32 



WILLIAM Q. LOEGERING 



Organism 

 Character 

 Phenotype 



Genotype 

 (Classical) 



Category JH Genetic Interaction 

 of a HostPathogen Relationship 



Host- 



Environment 



Aegri corpus Pathogen 



Reaction Infection Type Pathogenicity 



Genotype 

 (Gene-for-gene) 



Low High Low High Low High 



I I I I I I 



RR rr RRPP rrPP PP pp 



Rr RRPp rrPp Pp 



RrPP Rrpp 



RrPp RRpp 

 rrpp 



Lr Hr LrLp LrHp Lp Hp 

 HrLp 

 HrHp 



Figure 1. Diagrammatic representation of the gene-for-gene 

 interaction between a single set of corresponding gene pairs 

 in a host-pathogen relationship. Low reaction and pathogenicity 

 are shown as dominant for the sake of simplicity in the diagram. 







This interaction between corresponding gene pairs is referred to as 

 a Category III interaction (Loegering and Powers, 1962). (Categories I 

 and II are from classical genetics and represent, respectively, interaction 

 between alleles in one organism and interaction between genes at different 

 loci in one organism.) Sometimes low reaction and/or low pathogenicity 

 are recessive rather than dominant as shown in Figure 1. This does not 

 change the basic principle but if both are recessive only one aegricorpus 

 genotype (rrpp) will give low infection type. 



We can see then that a host gene pair for low reaction (whether 

 homozygous or heterozygous dominant or homozygous recessive) has the 

 potential to produce low infection type which can be designated Lr (where 

 L is for low and r is for reaction) , whereas a gene pair for high reaction 

 has the potential for high infection type and can be designated Hr. The 

 same is true for the pathogen with respect to the corresponding gene pair, 

 and we can have a genotype for low pathogenicity designated Lp or high 

 pathogenicity designated Hp. Each of the symbols refers to any combination 

 of alleles at one locus in one of the organisms which has the potential to 

 condition high or low infection type respectively. 



Using these symbols, we find there are only four potential genotypes 

 for the aegricorpus in a Category III interaction: Lp/Lr * Lit (low infec- 

 tion type), and Lp/Hr, Hp/Lr s and Hp/Hr = Hit. These can be compared to 



