The affix ḍa comes after the verb jan 'to be produced', with a past signification, when the word, in composition with it is in the Ablative -5th case, which does not denote a genus.,
As बुद्धिजः 'born through understanding'; संस्कारजः 'born of habit'; दुखजः 'born through pain'.
Why do we say 'when not denoting a genus'? Observe हस्तिनो जातः 'born of elephant'; अश्वाज्जातः 'born of horse'.,