The affix ḍhak comes in the sense of a descendant after a word ending in a feminine affix and consisting of two vowels.,
Thus दत्ताया अपत्यं = दात्तेयः 'son of Datta', गौपेयः \son of Gopi\. This debars the अण् of 4.1.113.
Why do we say 'having two vowels ?' Observe यामुनः 'son of यमुना', which is a trisyllabic word, and not dissyllabic.,