The affix ḍhak comes in the sense of a descendant after a disyllabic prātipadika (Nominal Stem) ending in short i , but not, however after a word ending in the Patronymic affix iñ,
The anuvritti of 'stri' does not extend to this aphorism. The force of the word च in the sutra is to draw the anuvritti of the word द्व्यचः of the last sutra. Thus आत्रेयः 'the son of Atri'. नैधेयः 'the son of निधिः' ।
Why do we say \after a word ending in short इ\? Observe दाक्षिः \son of दक्ष\ ॥ Here दक्ष though a dissyllabic word, yet ends in अ ॥
Why do we say 'not ending in the patronymic affix इञ्'? Observe दाक्षायणः 'son of दाक्षि' ॥ Here though दाक्षि is a dissyllabic word ending in short इ, it does not take the affix ढक् for इ here is the affix इञ् (IV. 1. 95.); so also प्लाक्षायनः ॥ Why do we say 'having two vowels'? Observe मारीचः \son of मरीचिः\,