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Analysis of Several Types of Gene Action Controlling Resistance to Bacterial Blight of Cotton by Means of a Generalized Computer Model¹

A computer program has been written having the capability to fit three- or four-gene models to F₂ segregation data divided into ten or fewer phenotypic classes. The various types of gene action analyzed are known to occur in the determination of bacterial blight resistance in cotton. The genes in any specific model may be independent or dependent, unequal in effect, and either dominant, partially dominant, additive, or recessive. Successive increments of gene values were limited to integers. Linkage was in five increments from 0 to 50% recombination inclusive, coupling or repulsion, and restricted to two genes in any particular model. The X² criterion was used to select those models printed out as possible solutions. Exact data based on hypothetical three-gene models were solved satisfactorily by the computer program. To provide a "live" test, a strain of cotton (Gossypium hirsutum, L.) resistant to the bacterial blight pathogen (Xanthomonas malvacearum [(E. F. Smith) Dowson], was crossed to susceptible strain. Data from the segregating progenies were analyzed by the program, and feasible models for the gene action accounting for resistance were proposed. Classification into eight grades of resistance-susceptibility were shown to be statistically satisfactory for resistant and susceptible parents and for a segregating progeny in a specified environmental range of temperature and humidity. Conclusions regarding gene action are restricted to the specified environmental range. The program is best adapted for analysis of samples of 200 to 350 individuals.

Key Words: Epistasis • Xanthomonas • Genetic recombination • Quantitative genetics

² Associate Professor, Department of Agronomy, New Mexico State University, Las Cruces, New Mexico, 88003; Computer Programmer, ARF Company, Boulder, Colorado; and former graduate assistant, New Mexico State University, now Assistant Professor, Department of Agronomy, North Dakota State University, Fargo, North Dakota, respectively.