Is it always the case that the firstborn resembles his father, and why?

We have not found anything, neither in religious texts nor in reality, to indicate that the firstborn male child resembles his paternal grandfather. Rather, religious texts indicate that a child resembles the one whose fluid preceded into the womb, and reality proves that many children do not resemble their fathers, such as Al-Hasan (definition) ibn Ali, who resembled the Prophet, peace and blessings be upon him, more than he resembled his father.