What is the smallest unit of descent in a society?
If you are referring to closest (genetic) descent it would be between the two biological parents and the child or children born of that union. However, there are societies where by the closest descent runs specifically through either a matriarchal or patriarchal line. In these cases only one set of genes is recognized as the direct line of descent. For example in a matriarchal line closest descent is between grandmother, her children, mother, her children and so on. The father and his ancestry is not considered part of the direct lineage.