chi-square test

Constructing and testing a gene-for gene model

A crop has two well characterized genes which control resistance to a fungal pathogen with known races 1-3.Line 1 has gene R1 and line 2 has gene R2. After testing a large number of fungal isolates with lots of germplasm, you find that you have two new races and six new sources of resistance.Table 1 shows the reactions of the five fungal races on the different lines that you have identified.Make a model for the genetics of resistance in these lines.Indicate which resistance genes you think lines C through H have and which avirulence genes you think each race has.

The data in Table 2 correspond to segregation ratios (resistant : susceptible) of F2 progeny made from a cross between the two parents given.All of the parental lines are homozygous.The Line U is a universal suscept (i.e. carries no known resistance genes).Use this genetic data to test your model and make any adjustments necessary.

TABLE 1

TABLE 2

Chi-Square Test

Chi-square test for segregation ratios:

Χ2 = ∑ [(Obs. – Exp.)2 / Exp.]

D.f. = number of classes – 1

Obs. = observed counts in each class

Exp. = expected counts in each class based on Mendelian inheritance (or recombination frequency-adjusted inheritance)

For each of the segregation ratios in Table 2 (only the ones with resistant plants), calculate a chi-square for the null hypothesis that the observed class frequencies equal the expected frequencies for a single R gene vs. two R genes.Adjust your model based on these tests.

Chi-Square Distribution