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?


closed as off-topic by rg255, AliceD, James, March Ho, fileunderwater Apr 22 '16 at 8:16

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    $\begingroup$ All of your grandparents could have been brown eyed. Your having blue eyes only requires that one grandparent on the maternal and one on the paternal side were carriers. Note though that eye colour is not as simple as dominant and recessive. $\endgroup$ – rg255 Apr 18 '16 at 5:25
  • $\begingroup$ Related: Is it possible for a brown eyed parent to have blue eyed child? $\endgroup$ – fileunderwater Apr 22 '16 at 8:16

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.

See also:


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.

No, this does not necessarily follow, because eye color genetics are more complicated than just one dominant-recessive gene. See, for example this article from a someone at Stanford University on How Blue Eyed Parents Can Have Brown Eyed Children. It isn't common, but apparently it does happen.

Solving your inheritance just based on eye color with 100 percent certainty is not possible.


On a high-school level you are right. Still, the answer to your question is no.

As you may have already noticed, nothing in reality is as simple as schools tell us. Eye colour is generally determined by one gene, but there are other genes which can modify the effect. Think about this question: if there were just two alleles, blue and brown, where would green-eyed and grey-eyed people come from?

If you want to investigate your who is your biologic father, you can ask them both to take a genetic test. If they decline or cannot comply, try comparing many traits, not just one. Ask for photographs of you mother and both possible fathers when they were your age and make a list. What is everybody's hair colour? What is their height? What is the shape of their faces? the proportions of bodies? the skin colour? shape of mouth, nose, eyes? Do you know everybody's blood type? Maybe you just look a lot like one of the men, then you can assume he's your father. Maybe you look a lot like your mother and there are only subtle differences which point to one of the men? Remember that this comparing does not give you certainty, only clues.


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