The Genitive 6th-Case-affix is retained after a word ending in ṛ and expressing a relationship through study or blood.,
Thus होतुरन्तेवासिन्, होतुः पुत्रः, पितुरन्तेवासिन् or पितुः पुत्रः ॥ Why do we say \a word ending in ऋ\? Observe आचार्यपुत्रः, मातुलपुत्रः ॥ The relationship through 'study' or 'blood' should be between the first and the second member of the compound. Therefore, the rule does not apply to होतृधनम्, पितृधनम्, होतृगृहम्, पितृगृहम् ॥,