  This paper considers the limiting distribution of $\pi_{\lambda,\theta}$, the
stationary distribution of the infinitely-many-alleles diffusion with symmetric
overdominance \cite{MR1626158}. In \cite{MR2519357} the large deviation
principle for $\pi_{\lambda,\theta}$ indicates that there are countably many
phase transitions for the limiting distribution of $\pi_{\lambda,\theta}$, and
the critical points are $\lambda=k(k+1), k\geq1$. The asymptotic behaviours at
those critical points, however, are unclear. This article provides a definite
description of the critical cases.
