Been long time since my high school biology. I know about XX/XY chromosomes, and I know about dominant and recessive genes, but I'm a bit rusty about the connection. If father has brown eyes, and son has blue eyes, it is a foregone conclusion that father has a brown-eyed X and a blue-eyed Y? That's the only way the son could be blue-eyed, right? Because he must have gotten his Y from his father, and it must have been blue.