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STABLISHMENT of an inversion arrangement of any chromosome consists E of three stages. The first stage is represented by stochastic survival of an inversion which has newly occurred in a given population. This stage is stochastic because a given inversion probably appears only in one individual in a population whether this population is large or small in size; and consequently the survival process of inversion descendants is subjected to stochastic errors. OHTA and KOJIMA (unpublished) examined such a stochastic process in large populations using a simplified genetic model.The second evolutionary stage of inversions is the phase in which the frequency of an inversion increases in a population. For a small population, this stage is also stochastic and cannot be distinguished from the first stage. However, once the number of inversion descendants becomes large in a large population, the chance elimination of all inversion descendants becomes extremely unlikely, and the second stage can be treated deterministically using the relative frequency of inversion descendants.The third stage concerns the genetic mechanism(s) by which inversions and their standard arrangements may be brought to a balanced polymorphism. Some conditions for this polymorphism were investigated by HALDANE (1957). This paper deals with the second evolutionary stage of inversions under a simple condition. In particular, the condition is that the selective advantage of an inversion is caused by a favorable combination of alleles in the inverted segment of a chromosome when the population is in mutation-selection equilibrium. The specific objective of this paper is to examine whether an inversion chromosome carrying few deleterious alleles relative to the average number of such alleles per chromosome can increase in frequency in such a population. Partially Recessive MutationsConsider a large random mating population in which selectively advantageous alleles, A's, and partially recessive deleterious alleles, a's, are maintained in the ' Paper Nii.

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