The genealogy of Jesus is traced through Joseph, but Joseph had nothing to do with Jesus' conception. The following explanation sheds light on what seems to be a confusing issue.
We have two genealogies of Jesus- Matthew 1:1-17 and Luke 3:23-38. Because there are so many substantial differences between these two (for example, many of the ancestral names don't match), they have given scholars a headache through the ages. For example: Who was Joseph's father? Was it Jacob (according to Matthew), or Eli (according to Luke)?
One answer is: both lists are family records, but Matthew is giving us Joseph's record, and Luke is giving us Mary's. But that answer goes against the text - Luke makes it clear that he is tracing Jesus' descent through Joseph. Nor does it fit with what we know of ancient middle eastern peoples. A genealogy traced through the mother would not have been normal at that time and place in history.
We have to remember that Israel's origin was tribal. The clan leader was, of necessity, a dominant male. The individual's survival depended on being able to claim membership within the tribe. Since in real life many things could happen to a bloodline, a number of supplementary laws and customs developed. A person could become a member of a clan without actually being born into it. One way was by adoption. Another was to be born of a woman who was married to a man of that clan. Even when the husband was not the child's biological father, he was still officially the legal father, simply because he was husband to the child's mother.
In the Bible, genealogies can serve different purposes. Besides establishing identity, they can also be used to structure history into epochs and to authenticate a line of office-holders. That's why an individual can be accorded two or more genealogies according to the purposes for which they were drawn up. Rarely do ancient biblical genealogies afford us a list of strictly biological ancestry.
What were Matthew's and Luke's purposes in giving Jesus a genealogy? They list different ancestors but agree totally on the most important fact: Joseph was not the biological father of Jesus. To see how Matthew made a strong statement about this, read slowly Matthew 1:1-17. Let the repeated, rhythmic phrases "A the father of B," "B the father of C," and so forth, almost lull you to sleep. What happens when you get to verse 16? The lilting, fixed pattern is suddenly altered: "Jacob was the father of Joseph the husband of Mary. It was of her that Jesus who is called the Messiah was born." By using his genealogical list in this way, Matthew was able to proclaim both that Jesus was virginally conceived and that he was also legitimately a "son of David, son of Abraham" (1:1). For Matthew's Jewish Christian audience this was like calling Jesus the Messiah.
Luke proclaims our Lord's virginal conception when he speaks about Jesus as being - so it was supposed - the son of Joseph in 3:23. He then takes his genealogy back to Adam and even to God himself. In doing this he is stating that Jesus is nothing less than the Son of God. Because neither evangelist was principally concerned with Jesus' biological ancestry, the lists could differ, and each evangelist could present a different popular tradition suitable to his own specific purpose.
So we see that Joseph was not Jesus' biological father, but he was his legal father. The two genealogies make that point emphatically. Because of that, all of us can now proclaim with the Scriptures that Jesus was, indeed, son of David, son of Abraham, and Son of God.
© Liguori Publications Excerpt from Advent - A Quality Storecupboard The Congregation of the Most Holy Redeemer