Topic
Hadith speak of children’s resemblance to parents and of a “vein” of ancestry, in the language of the time.
What it means
The Companion Abdullah ibn Salam asked the Prophet why a child resembles its father or its mother, and he answered that if the man’s fluid prevails the child resembles him, and if the woman’s, her (Sahih al-Bukhari). A man whose wife bore a dark child asked whether to disown it, and the Prophet asked about his camels, and whether any was grey; “perhaps a vein drew it”, he said, “perhaps your son’s vein drew him” (Sahih al-Bukhari, Sahih Muslim).
What the sources say
Muslim scholars read the first as an observation about the dominance of one parent’s traits, in the terms available then. The second sets out heredity across generations, and the Prophet’s point was that the child was not to be disowned on appearance. Neither is presented as a lesson in genetics.