Wiener kernels have been measured for the light growth response of a number of mutants of Phycomyces which show abnormal phototropism (mad mutants). Representative mutants were chosen from the six complementation groups (madA to madF) associated with the light response pathway. One group, madA, associated with the input part of the pathway, exhibits an essentially normal response provided it is tested above its moderate threshold. The groups madB and madC appear more defective, in that their kernel amplitudes are very small even above their thresholds. Their similarity to each other suggests a close functional connection between the respective genes. The remaining three groups (madD, madE, and madF) have all been associated with the output of the pathway. Tbe kernels for all three indicate a gain reduction, which depends gradually on intensity. These three groups appear to have the same absolute threshold as wild-type. None of the mutants studied shows special behavior at high intensity that could be evidence of alterations in the photoreceptor complex.
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