Final answer:
Unilineal descent is the term used when ancestry is traced through only one line, either the mother's or the father's line. It includes patrilineal and matrilineal descent, as well as ambilineal descent where the line chosen can vary.
Step-by-step explanation:
When a descent system traces ancestry through one line, whether it be the mother's or the father's, this is called unilineal descent. This system traces an individual's kinship through a single gendered line. There are three types of unilineal descent: patrilineal, which follows the father's line exclusively; matrilineal, which traces through the mother's side only; and ambilineal, which allows a choice between the father's or the mother's line depending on the situation. Patrilineal descent is commonly observed in rural China and India, with males carrying on the family surname, whereas matrilineal descent, tracing through female ancestors, is noted in certain cultural groups and societies.