Monosyllabic nouns of the third declension have the accent on the ultima in the genitive and dative of both numbers. In the genitive plural it is the circumflex.
Example: σάρξ, σαρκός, σαρκῶν
This rule is an exception to the rule of noun accent. In accordance with the rule of noun accent, the accent would remain on the same syllable as in the nominative singular so nearly as the general rules of accent would permit.