Genetik i universum verkar följa genuin genetik.
Alla exemplen du listar följer de dominerande egenskaperna hos sina föräldrar:
Jon Snow:
Parents are Lyanna Stark (mother) and Rhaegar Targaryen (father). Lyanna had dark hair (dominant) while Rhaegar had silver/blonde hair (recessive). Jon has dark hair.
Tyrion Lannister:
Parents are Joanna Lannister (mother) and Tywin Lannister (father) (yes, they were cousins). Both parents had blonde hair. Tyrion had blonde hair.
Alternate theory: Parents are Joanna Lannister and Aerys "The Mad King" Targaryen. This is a popular theory because Tyrion's hair color has been described as closer to silver, which is a trait of Targaryens. However, this would not affect the genetics, as both parents would still have the recessive trait.
Tommen Baratheon:
Parents are Cersei Lannister (mother) and Jaime Lannister (father). Tommen is not related to King Robert, despite having the Baratheon name. Both his biological parents have blonde hair. Tommen has blonde hair.
Kom också ihåg ur ett universellt perspektiv att skådespelare kanske inte har exakt hår eller ögonfärg som deras karaktärer beskrivs som i böckerna.