Sometimes the sons of brahmanas or ksatriyas become vaisyas (brahmana-vaisyatam gatah). When a ksatriya or brahmana adopts the occupation or duty of a vaisya (krsi-go-raksya-vanijyam (BG 18.44)), he is certainly counted as a vaisya

Expressions researched:
"sometimes the sons of brāhmaṇas or kṣatriyas become vaiśyas (brāhmaṇa-vaiśyatāṁ gatāḥ). When a kṣatriya or brāhmaṇa adopts the occupation or duty of a vaiśya" |"kṛṣi-go-rakṣya-vāṇijyam" |"he is certainly counted as a vaiśya"

Srimad-Bhagavatam

SB Canto 9

All the descendants of Ajamīḍha, who was a kṣatriya, became brāhmaṇas. This was certainly because of their qualities and activities. Similarly, sometimes the sons of brāhmaṇas or kṣatriyas become vaiśyas (brāhmaṇa-vaiśyatāṁ gatāḥ). When a kṣatriya or brāhmaṇa adopts the occupation or duty of a vaiśya (kṛṣi-go-rakṣya-vāṇijyam (BG 18.44)), he is certainly counted as a vaiśya. On the other hand, if one is born a vaiśya, by his activities he can become a brāhmaṇa.

SB 9.21.21, Translation and Purport

From King Hastī came three sons, named Ajamīḍha, Dvimīḍha and Purumīḍha. The descendants of Ajamīḍha, headed by Priyamedha, all achieved the position of brāhmaṇas.

This verse gives evidence confirming the statement of Bhagavad-gītā that the orders of society—brāhmaṇa, kṣatriya, vaiśya and śūdra—are calculated in terms of qualities and activities (guṇa-karma-vibhāgaśaḥ (BG 4.13)). All the descendants of Ajamīḍha, who was a kṣatriya, became brāhmaṇas. This was certainly because of their qualities and activities. Similarly, sometimes the sons of brāhmaṇas or kṣatriyas become vaiśyas (brāhmaṇa-vaiśyatāṁ gatāḥ). When a kṣatriya or brāhmaṇa adopts the occupation or duty of a vaiśya (kṛṣi-go-rakṣya-vāṇijyam (BG 18.44)), he is certainly counted as a vaiśya. On the other hand, if one is born a vaiśya, by his activities he can become a brāhmaṇa. This is confirmed by Nārada Muni. Yasya yal-lakṣaṇaṁ proktam. The members of the varṇas, or social orders-brāhmaṇa, kṣatriya, vaiśya and śūdra-must be ascertained by their symptoms, not by birth. Birth is immaterial; quality is essential.