SB 9.15.10
tad viditva munih praha
patnim kastam akarasih
ghoro danda-dharah putro
bhrata te brahma-vittamah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the great sage Rcika returned home after bathing and understood what had happened in his absence, he said to his wife, Satyavati, “You have done a great wrong. Your son will be a fierce ksatriya, able to punish everyone, and your brother will be a learned scholar in spiritual science.”
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
A brahmana is highly qualified when he can control his senses and mind, when he is a learned scholar in spiritual science and when he is tolerant and forgiving. A ksatriya, however, is highly qualified when he is fierce in giving punishment to wrongdoers. These qualities are stated in Bhagavad-gita (18.42-43). Because Satyavati, instead of eating her own oblation, had eaten that which was meant for her mother, she would give birth to a son imbued with the ksatriya spirit. This was undesirable. The son of a brahmana is generally expected to become a brahmana, but if such a son becomes fierce like a ksatriya, he is designated according to the description of the four varnas in Bhagavad-gita (catur-varnyam maya srstam guna-karma-vibhagasah). If the son of a brahmana does not become like a brahmana, he may be called a ksatriya, vaisya or sudra, according to his qualifications. The basic principle for dividing society is not a person’s birth but his qualities and actions.