Samples of asexual spores of a two-component heterokaryon of Neurospora crassa collected on Millipore filters or in suspension, were flown aboard the Gemini-XI spacecraft in September 1966 to permit an evaluation of the genetic effects of radiation in combination with weightlessness. A ground-based experiment was performed simultaneously as a control. The samples were exposed to different amounts of32 P β-irradiation and survival levels and forward-mutation frequencies in the ad-3 region were studied. Characterization of the ad-3 mutants has been performed by a series of genetic tests to determine the spectra of genetic alterations in the in-flight and ground control samples. These tests have shown that there is no difference between the spectra of genetic alterations in the in-flight and ground samples irradiated on filters. These tests have also shown that the lower frequencies of forward-mutation in the in-flight samples irradiated in suspension was due to a specific effect on point mutations (<tex-math>$ad\text{-}3^{{\rm R}}$</tex-math>) and not chromosome deletions (<tex-math>$ad\text{-}3^{{\rm IR}}$</tex-math>). All classes of point mutations showed this same effect. These results on the spectrum of ad-3 mutation are quite different from those expected from anoxia. We conclude that the higher levels of survival and lower frequencies of forward-mutation in the in-flight samples in suspension are due to protection against the genetic effects of radiation by weightlessness.

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