Mbh 13.48.2c

Persons belonging to the mixed castes beget upon spouses taken from their own castes children invested with the status that is their own. When they beget children in women taken from castes that are inferior to theirs, such children become inferior to their fathers, for they become invested with the status that belongs to their mothers. Thus as regards the four pure orders, persons beget children invested with their own status upon spouses taken from their own orders as also upon them that are taken from the orders immediately below their own. When, however, offspring are begotten upon other spouses, they come to be regarded as invested with a status that is, principally, outside the pale of the four pure orders. When such children beget sons in women taken from their own classes, those sons take the status of their sires. It is only when they take spouse from castes other than their own, that the children they beget become invested with inferior status. As an example of this it may be said that a Sudra begets upon a woman belonging to the most superior order a son that is outside the pale of the four orders (for such a son comes to be regarded as a Chandala who is much inferior). The son that is outside the pale of the four orders by uniting with women belonging to the four principal orders, begets offspring that are further degraded in point of status. From those outside the pale of the four orders and those again that are further outside that pale, children multiply in consequence of the union of persons with women of classes superior to their own. In this way, from persons of inferior status classes spring up, altogether fifteen in number, that are equally low or still lower in status. It is only from sexual union of women with persons who should not have such union with them that mixed classes spring up.