I know all my family up to my grandparents, with the exception of the grandfather on my mother's side. At the time my mother was conceived, my grandmother was having an affair so no one in my family knows for sure who the grandfather was.
In high school we learnt that blue eyes are a recessive trait and brown eyes are a dominant trait, as such I should be able to figure out who my grandfather is, but I only studied up to that level and I want to check with someone more knowledgeable than myself if it is correct.
So here's my reasoning: I have blue eyes, meaning I have two blue eyed genes and got one from my father and one from my mother. My mother has brown eyes, meaning that she has one brown eyed gene and one blue eyed gene to pass it on to me. My grandmother has blue eyes, so she passed one on to my mother. One of my potential grandfathers had blue eyes, so if it had been him my mother would have received two blue eyes genes and have blue eyes. But she has brown eyes so it must be the other potential grandfather (who's eyes I don't know the colour of). Can someone tell me if this is correct, and if not, why not?
Answer
You are trying to work out who your mother's father was. You know that your mother's mother had blue eyes, but your mother had brown eyes. You also have blue eyes. You make an assumption that eye colour is a Mendelian trait, with a dominant brown allele (A) and recessive blue allele (a) such that AA and Aa are brown-eyed, while aa gives blue eyes.
To have blue eyes you must have gotten an a from each parent, so your mother (who is brown-eyed) must have been Aa. You know that her mother was aa, because she had blue eyes. Your mother must have inherited her A from her father, therefore, your mother's father had brown eyes.
Can this information identify your grandfather? Maybe, depending on the list of candidates (e.g. you have two men and you know that one of them is the grandfather, and one has blue eyes, the other has brown eyes). However, it's based on an assumption that eye colour is a Mendelian trait, with a dominant brown-eye allele and recessive blue-eye allele, and that a mutation has not occurred which gave your mother brown eyes. That assumption is flawed, eye colour is not Mendelian (it's polygenic), and you cannot rule out mutation in eye colour genes. That said, you are more likely to resemble the people you are more closely related to, so resemblance in eye colour could offer some clues (as would other easily visible characteristics like skin type, hair colour, hair type, facial features, height, physique). In short, you cannot identify your biological grandfather by eye colour, but it may give some slight indication.
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