Professor,

Joseph and Mary came from the line of David. Joseph was
Our Lord's foster father so it is very important that Mary
descended from David.

I posted the same at Jesus on the Cross thread.

Numbers 36:6 And this is the law promulgated by the Lord touching the daughters of Salphaad: Let them marry to whom they will, only so that it be to men of their own tribe. 7 Lest the possession of the children of Israel be mingled from tribe to tribe. For all men shall marry wives of their own tribe and kindred: 8 And all women shall take husbands of the same tribe: that the inheritance may remain in the families, 9 And that the tribes be not mingled one with another, but remain so 10 As they were separated by the Lord. And the daughters of Salphaad did as was commanded:

11 And Maala, and Thersa, and Hegla, and Melcha, and Noa were married to the sons of their uncle by their father, 12 Of the family of Manasses, who was the son of Joseph: and the possession that had been allotted to them, remained in the tribe and family of their father.