Though he does not center a good portion of his narrative on Davidic descent as Matthew does, Luke does clearly present the Son of God as the Davidic Messiah. Again it is through Joseph that Jesus is linked to David. The Protoevangelium of James and others since have attributed Davidic descent to Mary, but Luke never does so, and the fact that she is kin to Elizabeth of the daughters of Aaron (Lk 1:5,36) could be argued as placing her also in Aaron's rather than in David's line. In Luke 1:27 the phrase "of the house of David" which grammatically follows Joseph's name refers to him alone, as is consistent with Luke 2:4 and 3:23. Joseph is introduced as important for introducing Mary. Before hearing the angel's words to her, the reader must know that she is a virgin betrothed to a man of the house of David. Joseph's role is what makes understandable the announcement that the Son of God conceived in Mary will be given "the throne of his father David, and he will reign over the house of Jacob for ever" (Lk 1:32-33).
The birth narrative similarly stresses that Joseph went for the census to Bethlehem, the city of David, "because he was of the house and lineage of David" (Lk 2:4). From their home in Nazareth, Mary journeys with him so that the child may be born in the city of David. Luke emphasizes Bethlehem more than Matthew, and the first announcement to the shepherds is that "in the city of David" is born a Savior, who is the Messiah, the Lord (Lk 2:11).
Luke's genealogy follows the infancy narrative and Jesus' baptism by John at the beginning of his ministry. John has told the people that being a descendant of Abraham is not sufficient for salvation, and that God's salvation is not limited by ancestry, for he can raise up children of Abraham from stones (Lk 3:8). At Jesus' baptism he is manifested as the beloved Son of heaven accompanied by the Holy Spirit (Lk 3:21-22). The genealogy sums up these two themes of human ancestry and divine Sonship by going back beyond Abraham, all the way to Adam and ultimately to God (Lk 3:38). Jesus initiates a course of history proceeding not only from Israel, but from humanity and from God Himself. Salvation is to be directed to all the children of Adam, equally created by God. Jesus' genealogy is traced through Joseph.
Jesus was "the son (as was supposed) of Joseph" (Lk 3:23). The "as was supposed" refers to the public being unaware of the virginal conception (Lk 1:27,34-37) and their considering Joseph to be the biological father, and to their not seeing beyond the human father to the divine Father of whom Jesus is Son in a more profound sense. Luke has portrayed Joseph as the transmitter of Davidic descent to Jesus quite apart from biological parenthood. Besides being the reputed father of Jesus, Joseph is the legal, genealogical father through whom Jesus traces his lineage.
Though the genealogy contains no markers, comments, or subdivisions, it has been shown to contain eleven groups of seven names, much as Matthew's three groups of fourteen could be broken into six groups of seven (the perfect number). Jesus has 77 ancestors listed, the last of whom is God. The names falling in the positions which are multiples of seven are significant, numbers 7 and 35 being two other Josephs. This list culminates with God whose number 77 is doubly perfect. It includes Abraham and David, the names given such importance by Matthew. It would seem that while using quite distinct forms, Matthew and Luke both emphasize Abraham, David, Joseph, and multiples of the number seven.
While agreeing on Davidic descent, Luke differs from Matthew in showing this descent to occur through Nathan rather than Solomon. What is certain is that it is through Joseph that Jesus is son of David and son of Adam, the first human. The Son of God (Lk 1:35; 3:22,38) traces his human ancestry all the way back through creation by means of Joseph.
Taken from http://www.osjoseph.org/stjoseph/scripture/part2sec2.php
