ואם אין לו בת ונתתם את נחלתו לאחיו
And if he has no daughters, you shall give it to his brothers
ואם אין לו אחים ונתתם את נחלתו לאחי אביו
And if he has no brothers you shall give his inheritance to his father's brothers.The question is that the father has the right of inheritance before the paternal uncles. The Ramban suggests that the Torah does not want to explicitly state that such a tragedy has taken place. A parent should never have to bury his child. The Torah often goes out of its way to teach a law in using expressions of blessing rather than bad fortune even when referring to a law that comes about because of something sad (as in this case referring to a death).
While there is a hint in the Torah in 27:8, where the term used is pass over, the law actually cannot be stated explicitly. The case being considered is regarding the inheritance of the portion of land being assigned to the family. If the father were still alive, then the son would never have gotten such a portion of land. Any land that he had bought would be subject to returning to it's original owner at Yovel. It would not be part of the נחלה, inheritance, though the heirs would inherit the lease. Thus, the case could not explicitly speak of a father inheriting from a son.