Mark 12:18-19

‘And there come to him Sadducees who say that there is no resurrection, and they asked him saying, “Teacher, Moses wrote to us that if a man’s brother die, and leave a wife behind him, and leave no child, that his brother should take his wife and raise up seed to his brother.” ’

This provision of the Law is found in Deuteronomy 25:5-10. The purpose of it was to ensure that land remained within a family, and to ensure continuity of the line. Men lived on in their children. Thus it was looked on as a brotherly duty to ensure that a man who died without children had children provided by the seed of his brother being planted in his surviving wife. The child was then looked on as being the child of the dead man and inherited accordingly.