The answer is two-fold.
First, Jesus was adopted by Joseph, the husband of Jesus' mother, Mary. So, while Jesus was not conceived by Joseph, he was the son of Joseph. Joseph was a descendant of Abraham (see Matthew 1:1-16).
Second, Jesus was a physical descendant of Abraham because he was conceived of Mary. Mary, whose father was Eli, was a descendant of Abraham (Luke 3:23-38).
Zakir's assumption that Abraham can only have descendants through males is simply mistaken.
We see that descent was also traced through daughters. This can be seen in the case of Zelophehad:
Numbers 26:33 And Zelophehad the son of Hepher had no sons, but daughters: and the names of the daughters of Zelophehad were Mahlah, and Noah, Hoglah, Milcah, and Tirzah.
Then came the daughters of Zelophehad, the son of Hepher, the son of Gilead, the son of Machir, the son of Manasseh, of the families of Manasseh the son of Joseph: and these are the names of his daughters; Mahlah, Noah, and Hoglah, and Milcah, and Tirzah. And they stood before Moses, and before Eleazar the priest, and before the princes and all the congregation, by the door of the tabernacle of the congregation, saying, "Our father died in the wilderness, and he was not in the company of them that gathered themselves together against the Lord in the company of Korah; but died in his own sin, and had no sons. Why should the name of our father be done away from among his family, because he hath no son? Give unto us therefore a possession among the brethren of our father."So, daughters as well as sons are the descendants of Abraham, and it is sufficient for Jesus that he was a son of Mary.
And Moses brought their cause before the Lord.
And the Lord spake unto Moses, saying, "The daughters of Zelophehad speak right: thou shalt surely give them a possession of an inheritance among their father's brethren; and thou shalt cause the inheritance of their father to pass unto them. And thou shalt speak unto the children of Israel, saying,
- If a man die, and have no son, then ye shall cause his inheritance to pass unto his daughter.
- And if he have no daughter, then ye shall give his inheritance unto his brethren.
- And if he have no brethren, then ye shall give his inheritance unto his father's brethren.
- And if his father have no brethren, then ye shall give his inheritance unto his kinsman that is next to him of his family, and he shall possess it: and it shall be unto the children of Israel a statute of judgment, as the Lord commanded Moses.
Of course, it is enough for us that the Scriptures describe Jesus this way:
Matthew 1:1 The book of the generation of Jesus Christ, the son of David, the son of Abraham.
Galatians 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Even if we could not explain how this is so, we would accept it. But, as I've shown above, we can explain how.