Breaking down the question
The question asks you to distinguish two of the central organising principles of kinship — descent and alliance — and to illustrate the difference. These are not merely two topics but two rival theoretical approaches to how kinship systems are held together, associated respectively with the British structural-functionalists and with the French structuralist Lévi-Strauss.
The essential contrast is between the vertical tie of descent, which links generations through parentage, and the horizontal tie of alliance, which links groups through marriage. Descent theory sees the lineage as primary; alliance theory sees the exchange of spouses between groups as primary. Bringing out both the substantive and the theoretical difference is the key to a strong answer.
How to approach it
Define descent and alliance clearly and place each within its theoretical home — descent with Radcliffe-Brown and Fortes, alliance with Lévi-Strauss. Then draw the contrast under a few heads: the axis of relationship (vertical versus horizontal), the basis of the tie (blood versus marriage), and what each treats as the foundation of social structure.
Illustrate throughout, using unilineal descent groups on one side and prescriptive marriage systems on the other. The organisation of marriage and household is set out in our notes on family, household and marriage. At 10 marks, keep the framework tight and the illustrations brief.
Model answer
Descent and alliance are two fundamental principles by which kinship systems are organised, and they correspond to two influential theoretical traditions in social anthropology. A descent rule specifies how membership of a kin group is transmitted across generations, tracing relationship through parentage. An alliance rule concerns the relationships created between groups through marriage. The first is a vertical, consanguineal tie of blood; the second is a horizontal, affinal tie of marriage.
Descent operates through the line of parentage and defines who belongs to a given kin group. It may be unilineal — patrilineal, tracing descent through the father, or matrilineal, through the mother — or cognatic, reckoned through both. Descent groups such as lineages and clans regulate the inheritance of property, name, office and ritual status. The descent approach, developed by A. R. Radcliffe-Brown and Meyer Fortes in the British structural-functionalist tradition, treats the unilineal descent group as the basic building block of society, providing continuity, corporate identity and the framework of jural rights and duties. The patrilineal joint family of much of northern India, in which membership, surname and ancestral property pass through the male line, illustrates the descent principle at work.
Alliance, by contrast, concerns the ties forged between groups through marriage. Here the crucial relationship is not that between parent and child but that between wife-givers and wife-takers. The alliance approach, associated with Claude Lévi-Strauss, holds that kinship structure rests on the exchange of women between groups, so that marriage is a form of reciprocity that binds otherwise separate descent groups into a wider social order. On this view the incest taboo compels groups to marry out, and the resulting exchange of spouses is the true foundation of social solidarity. Systems of prescriptive or preferential marriage, such as cross-cousin marriage in parts of south India, illustrate the alliance principle, since they establish enduring, repeated marital ties between particular groups.
The two therefore differ on several axes. Descent is vertical and consanguineal, linking generations by blood; alliance is horizontal and affinal, linking groups by marriage. Descent theory locates the source of social structure in the corporate lineage; alliance theory locates it in the exchange of spouses between lineages. Descent emphasises continuity and inheritance down the generations; alliance emphasises reciprocity and integration across groups.
The two principles are not mutually exclusive but complementary. Every kinship system uses descent to constitute its groups and alliance to relate them to one another; a lineage must both perpetuate itself through descent and reproduce itself by marrying outside its bounds. The theoretical debate concerns which principle is analytically prior, but in practice descent and alliance together weave the fabric of kinship — the one running vertically through the generations, the other horizontally between groups.
Examiner's perspective
Examiners reward candidates who see that this is a contrast of two theoretical traditions, not just two words. The strongest answers pin descent to Radcliffe-Brown and Fortes and alliance to Lévi-Strauss, and then draw the difference along clear axes — vertical versus horizontal, blood versus marriage, lineage versus exchange — with an illustration for each.
The commonest weakness is defining descent well but treating alliance merely as "marriage" without its structuralist meaning of exchange between groups. Illustrations such as the north-Indian patrilineage and south-Indian cross-cousin marriage are highly effective. A closing note that the two principles are complementary rather than opposed shows the analytical maturity the question invites.