  The current understanding of supernova and gamma-ray burst events suggests
important effects on the biosphere if one of more of them happened to strike
the earth in the past. In this paper we evaluate the possibility that life
extinctions which probably occurred due to excess of radiation occurring in the
geologic past might have left a genetic signature on surviving species. We
emphasize the signatures of these extinctions, proposing a quantitative model
to evaluate the surviving probability of the species, based on kinetic aspects
of the frequency of mutations and the DNA repair rate.
