Because gene-for-gene interactions between plants and parasites depend on the combinations of individual genotypes from both species, a formal genetic analysis can test individual host parasite genotypic interactions When the virulence segregation of the progeny from individual crosses repeatedly follows the Mendelian law, it is major evidence of the establishment of a gene-for-gene relationship. With the recent progress in genome-wide molecular markers, genetic linkage maps of the BPH will also support the sophisticated genetic analysis and provide a systematic procedure for identifying virulence factors in the BPH genome Care should be taken to ensure that the variations in the virulence-related genotypes within the population
are not an obstacle in the formal genetic analysis