I read the hazel-eye comments but they were irrelevant. I told the story of my sister and her husband and their kids and their grandfather, using them as a concrete example. But like an nosy old woman looking for conflict and interested in abusing others, you imputed motives all through my post that were not there and never were issues.
I did not say my brother in law had any lack of understanding of the science of Mendel or any other science. YOU, however, very rudely and out of the blue, with NO basis for your accusations, accused my brother in law of not understanding science. You had NO basis for this. I'm correcting you because you are accusing people YOU DO NOT KNOW of not knowing the most basic elements of science. You are wrong. You probably don't want to take this any further because you are going to continue being wrong and being corrected no matter how many times you try to cover up your old-womanish busybodying that has NOTHING to do with anyone you know. Don't insult my relatives, particularly with baseless charges. Go insult your own relatives. Mine aren't ignorant.
Look, here is your statement:
"My sister has blue eyes and so does her husband. But her son has brown eyes and it's because one of her parents (my dad) had brown eyes."
Now, to have truely blue eyes, one must have two recessive blue allels --- bb.
Sister = bb
Bro-in-law = bb
You will only get blue eyes. Period.
In the case of your sister having true blue eyes, the brown color of the your and your sister's father's eyes is irrellevant, other than as a statistical oddity.
(The reason for this is that each sperm carries only 1/2 of the DNA set of the father, as does each egg, albeit for the mother.)
Now you mention your sister's father having brown eyes.
Brown can be expressed BB, Bb, or bB.
The circumstances indicate to me that your sister is Bb --- which generally results in brown eyes ---- but can result in Hazel.
Here your sister passed on the "B," which took the normal course of being dominant and resulted in a brown-eyed kid, instead of your sister's hazel.
Oh, and before you accuse people of "not knowing the most basic elements of science," you might want to read their profile.