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Simulation of the rapid expansion of a breeding isolate starting with 20 or 40 founders indicates that a lethal gene present in one original founder can attain polymorphic frequencies up to .10. Migration into the isolate decreases the probability of a lethal gene attaining polymorphic frequencies, but these still occur at rather high frequencies. A model of population expansion through fission and the establishment of new popula­tions by a few founders also shows that the entire set of populations can attain an average gene frequency that is minimally polymorphic.