Unless otherwise provided, when any property shall be given or any appointee appointed under a will, or under a trust of which the decedent is a grantor and which by its terms becomes irrevocable upon or before the grantor's death, to any issue of a grandparent of the decedent and that issue dies before the decedent, or dies before that issue's interest is no longer subject to a contingency, leaving descendants who survive the decedent, those descendants shall take that property or appointment as the predeceased issue would have done if the predeceased issue had survived the decedent. If those descendants are all in the same degree of kinship to the predeceased issue they shall take equally or, if of unequal degree, then those of more remote degree shall take by representation with respect to the predeceased issue.