Manava Dharmashastra 4.245

4.245

uttamāngacchaṃstaiḥ saha sambandhaṃ kurvanbrāhmaṇaḥ śreṣṭhatāmeti | brāhmaṇagrahaṇaṃ kṣatriyavaiśyayorapi pradarśanārtham | pratyavāyena viparītācaraṇena hīnaiḥ saha sambandhena prātilomyena śūdratā gacchati | jāteranapāyāt tattulyatāṃ prāpnotītyuktaṃ bhavati |

Medhātithi’s commentary (manubhāṣya):
‘Having recourse to superior people;’—i.e., forming connections with them,—‘the Brāhmaṇa attains eminence.’ The term, ‘brāhmaṇa,’ standing for the Kṣatriya and the Vaiśya also.

‘By the contrary procedure,’—by adopting the opposite process—i.e., by forming connections with inferior people—the man becomes a Śūdra. Since it is not possible for the caste of a man to disappear, what is meant is that he becomes equal to the Śūdra.—(245)