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Make a gene-for-gene model for the data below. When making your model you want to
propose as few resistance genes as possible to explain your data. Figure 1 shows the
reactions of the different lines to the various pathotypes.
Resistance or susceptible reactions for five host lines with the four pathotypes
Host line Pathogen isolates
1 2 3 4
Line A R S S R
Line B S R S R
Line C S S R R
Line D R S R R
Line U S S S S
F2 populations from the following crosses were constructed by making the F1 hybrids listed,
growing the F1 plants and collecting self-fertilized seed. These F2 populations were inoculated with
the different pathotypes and the following segregation ratios were observed.
F2 family isolate used Segregation
Line A X Line U 1 70:29
(resistant:susceptible)
Line B X Line U 1 0:10
Line C X Line U 1 0:10
Line D X Line U 1 75:24
Line B X Line U 2 77:23
Line C X Line U 3 80:29
Line D X Line U 3 78:25
Line A X Line U 4 70:24
Line B X Line U 4 76:23
Line D X Line U 4 190:12
Line A X Line D 1 150:0
Line A X Line D 3 97:31
Line A X Line D 4 153:0
Line B X Line D 1 84:29
Line B X Line D 2 102:32
Line B X Line D 4 198:2
Line C X Line D 1 91:30
Line C X Line D 3 125:0
Line C X Line D 4 121:0
Are these segregation ratio’s expected from your model, or do you have to modify your model?
What are the expected segregation ratios from progeny of test cross families (crossed to a universal
suscept) made from F1 crosses between the two host genotypes given, when challenged with the
pathogen genotype given. Assume resistance and avirulence are completely dominant at each locus
and, as predicted by the gene-for-gene model, any resistance gene that is matched by an avirulence
gene conditions resistance.
Progeny segregation ratios
Parent 1 Parent 2 Pathogen genotype T.C. ratio F2 ratio
r1/r1, R2/R2, r3/r3 r1/r1, r2/r2, r3/r3 avr1, Avr2, Avr3
r1/r1, R2/R2, R3/R3 r1/r1, r2/r2, r3/r3 avr1, Avr2, Avr3
R1/R1, r2/r2, r3/r3 r1/r1, r2/r2, r3/r3 Avr1, avr2, Avr3
R1/R1, R2/R2, r3/r3 r1/r1, r2/r2, r3/r3 avr1, avr2, Avr3
r1/r1, R2/R2, r3/r3 r1/r1, r2/r2, r3/r3 Avr1, Avr2, avr3
R1/R1, R2/R2, r3/r3 r1/r1, r2/r2, r3/r3 avr1, avr2, Avr3
R1/R1, R2/R2, r3/r3 r1/r1, r2/r2, r3/r3 Avr1, Avr2, Avr3
r1/r1, R2/R2, R3/R3 r1/r1, R2/R2, R3/R3 Avr1, avr2, Avr3
R1/R1, r2/r2, r3/r3 r1/r1, r2/r2, r3/r3 avr1, Avr2, avr3
R1/R1, r2/r2, r3/r3 r1/r1, r2/r2, R3/R3 Avr1, Avr2, Avr3
R1/R1, r2/r2, r3/r3 r1/r1, R2/R2, R3/R3 Avr1, Avr2, Avr3
R1/R1, R2/R2, r3/r3, r4/r4 r1/r1, r2/r2, R3/R3, r4/r4 Avr1, avr2, Avr3, Avr4
R1/R1, R2/R2, r3/r3, R4/R4 r1/r1, r2/r2, R3/R3, r4/r4 Avr1, avr2, Avr3, avr4
R1/R1, R2/R2, r3/r3, r4/r4 r1/r1, r2/r2, R3/R3, r4/r4 Avr1, avr2, avr3, Avr4
R1/R1, R2/R2, r3/r3, R4/R4 r1/r1, r2/r2, R3/R3, r4/r4 Avr1, avr2, avr3, avr4
R1/R1, r2/r2, r3/r3, R4/R4 r1/r1, r2/r2, R3/R3, r4/r4 avr1, avr2, Avr3, Avr4
R1/R1, r2/r2, r3/r3, R4/R4 r1/r1, r2/r2, R3/R3, r4/r4 avr1, avr2, Avr3, avr4
More gene-for-gene models for practice
Plant Line Pathogen Isolates
Isolate 1 Isolate 2 Isolate 3 Isolate 4
Line A I C C C
Line B C I C I
Line C C C I C
Line D I C I C
Plant Line Pathogen Isolates
Isolate 1 Isolate 2 Isolate 3 Isolate 4
Line A C I C C
Line B C I C I
Line C C C I C
Line D I I I C
Plant Line Pathogen Isolates
Isolate 1 Isolate 2 Isolate 3 Isolate 4
Line A I C I C
Line B C I C I
Line C C I I I
Line D I I I C
