Read the 2 genealogies in Mat 1 and Luke 3 and compare. Notice in Matthew that the seed line goes through David to Solomon and goes on till Jeconiah. This same man was called by 2 other names in Scripture. He was also called "Coniah" and "Jehoichin". In Jeremiah 22:30 God was angry with this same man and cursed him. God pronounced that NONE of his descendants would prosper. Being in the line of Jesus would be a problem for him by blood.
Now look at Luke 3:23. We see that Joseph was declared to be the son of Heli. Matthew 1:16 declares Joseph was born of Jacob. God would not recognize Joseph as anything legal in the line of Jesus as he came through Jeconiah who was cursed. So how would Joseph be said to come from Heli, who was actually Mary's father?
Zelophehad is mentioned in Numbers 36 dealing with the problem of a man that was in line to inherit property in the Promised Land but only had daughters and no sons to pass it on to. Moses consulted God and God told him to make a statute found in Numbers 36:6-13. It says that the women can marry who they want as long as they marry within their fathers tribe. When they marry this way, their husbands would be adopted and could hold property for the woman. Joseph would have been cursed according to his relationship to Jechoniah, but when he married Mary, Joseph became legally the son of Heli and could inherit property for her. Joseph couldn't be the blood donor to Jesus due to the curse, but he was the legal father and husband to Mary by adoption. The Holy Spirit supplied the male DNA and Mary provided the egg for Jesus and it could be traced to the line of David on both sides both legally and biologically.
According to Scripture Jesus had to be from the line of David and He qualified through Solomon and Nathan.