SB 11.20.2
varnasrama-vikalpam ca
pratilomanulomajam
dravya-desa-vayah-kalan
svargam narakam eva ca
Translation:
According to Vedic literature, the superior and inferior varieties found in the human social system, varnasrama, are due to pious and sinful modes of family planning. Thus piety and sin are constant points of reference in the Vedic analysis of the components of a given situation — namely the material ingredients, place, age and time. Indeed, the Vedas reveal the existence of material heaven and hell, which are certainly based on piety and sin.
Purport:
Pratiloma indicates the combination of a superior woman with an inferior man. For example, the vaidehaka community consists of those born of a sudra father and brahmana mother, whereas the sutas are those born from a ksatriya father and a brahmana mother or from a sudra father and ksatriya mother. Anuloma indicates those born from a superior father and inferior mother. The murdhavasikta are those born of a brahmana father and ksatriya mother. Ambasthas are those born from a brahmana father and vaisya mother, and they often become medical men. Karana indicates those born of a vaisya father and sudra mother or of a ksatriya father and vaisya mother. That such mixing of castes is not very much appreciated in the Vedic culture is demonstrated in the First Chapter of Bhagavad-gita. Arjuna was very worried that the death of so many ksatriyas on the battlefield would lead to the mixing of superior women with inferior men, and on those grounds he objected to fighting. In any case, the entire Vedic social system is based on distinguishing between piety and sin, and Sri Uddhava is encouraging the Lord to explain more elaborately His statement that one should transcend both piety and sin.