Zechariah was Iddo's son.

"Then the prophets, Haggai the prophet, and Zechariah the son of Iddo, prophesied unto the Jews." Ezra 5:1

"Through the prophesying of Haggai the prophet and Zechariah the son of Iddo." Ezra 6:14

Zechariah was Iddo's grandson.

"Came the word of the Lord unto Zechariah, the son of Berechiah, the son of Iddo." Zech.1:1

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Explanation:

It is commonplace in the Bible to refer a person as a son of a notable ancestor. For example, the Jews are collectively called the “sons of Abraham”, and Jesus was often called the “Son of David”(the king). So, in genealogical listings in the Scripture, it is OK to skip a generation. In Zechariah’s case, Iddo was a close ancestor who was probably distinguished in the Israelite community. Berechiah was the father; Iddo was the grandfather of Zechariah.

"And Jacob called unto his sons, and said, Gather yourselves together, that I may tell you that which shall befall you in the last days... Dan shall judge his people, as one of the tribes of Israel. Dan shall be a serpent by the way, an adder in the path, that biteth the horse heels, so that his rider shall fall backward. I have waited for thy salvation, O LORD." Genesis 49: 1, 16-18