Eli5: is it possible for a single grandparent to pass on their eye colour to their grandchild?


Me and my partner both have brown eyes…
My side of the family have all brown eyes so I’m assuming we don’t have the blue eye gene.
My partners parents: his dads side have all brown eyes but his mother and aunts and grandfather have blue eyes. So my husband must be a carrier.
Will my brown eyed genes dominate his blue eye genes or is it possible for a blue eyed baby?
It’s so interesting how it works

In: Biology

Yes. Eye color is determined by a number (5 or more) of genes that you can still be carrying even if not all are expressed. So your eyes might be brown because there are active, your kids might be blue because all others express in them.

Brown eyes genes are dominant to blue eye genes. This means that if a person has both brown and blue eye genes they are going to have brown eyes. Therefore you can’t really tell if your side of the family has blue eye genes or not. Any person’s blue eye genes will be concealed if they also have brown eye genes, but they can still show up down the line
If they happen to match with their partner’s blue eye genes. So in short it’s possible that you both carry a combination of brown eye and blue eye genes. In that case the probability of having a blue eyed child is 1/4. Then again, if at least one of you only has brown eye genes, which is also entirely possible, then the probability of having a blue eyed child is zero (bar random mutation).

For all you know, so far, several of your ancestors may be carriers, perhaps even you. If you are, a blue eyed child is more likely. It’s also not redundantly coded, so any white person is one random mutation away from being a carrier. It is cross linked to skin color, because both involve melanin production, so if your family is black it’s much less likely.

it is possible, while genes for brown (and green) are dominant, you usually also carry the recessive information for blue eyes in your DNA.

if you google “eye colour genetics chart” for example you find a couple of images showing the likelyhood of a baby having a specific eye color dependant on their parents eye colors.

Yes! This concept can be explained using a simple method called [Punnet squares](https://genetics.thetech.org/ask/ask59).

In genetics, a person with blue eyes has a “bb” genetic code. A person with brown eyes has a “BB” genetic code. So what happens when a “BB” and a “bb” have a baby? The baby gets a “Bb” genetic code, taking one letter from each.

That baby will have brown eyes… *but* that little b does do something. It gives the possibility of a blue-eyed baby!

Let’s say a brown-eyed person with a Bb code and a blue-eyed person with a bb code have a baby. What are the possible outcomes? A Bb (brown-eyed) or a bb (blue-eyed)!

The same could happen if a brown-eyed person with a Bb code has a baby with a brown-eyed person with a Bb code. The baby happens to inherit a b from each parent. Now it’s a bb… blue-eyed baby!

Both parents have brown eyes. Where did this blue-eyed baby come from? From the little b each parent carried.

One important note: Most babies have blue eyes at birth, but they can darken naturally over time. So if you have a blue-eyed baby at birth, it can take a few months, or even a couple years, before you know the true color.

Eye color is influenced by multiple genes but there’s one main gene causing brown eyes. The brown eyes gene has dominant alleles which means that you only need to inherit it from one of your parents to have brown eyes. You can still inherit the recessive “non-brown” allele from your other parent but it won’t affect your eye color.

If two parents have one dominant brown eye allele and one recessive non-brown eye allele, their child can inherit only the non-brown eye alleles and since there’s no dominant allele present, the child won’t have brown eyes.

Entirely possible. If both you and your husband carry blue eyes genes (which do not show because brown is dominant) they can be passed on to your kid. My brother has blue eyes, for example, and the only other person in the family with blue eyes was my grandpa’s sister. But the genes still run in the family, it is just highly improbable but definitely not impossible to pass only blue eyes genes.

The default color of the eyes is blue. However most people have the gene that makes the brown pigments so the eyes turn brown. This does mean that if you have two copies of the gene for brown eyes then all your kids will have this gene and will get brown eyes. There are however two assumptions which could be wrong here. Firstly it is very hard to figure out if you you have two versions of the gene or only one just by looking at the color of the eyes of your family. It is possible that many of them also only have one copy of the gene and you would never know. So there is a possability for you to birth kids with blue eyes for this reason. Another assumption is that the gene for brown eyes actually activates in your kids. It is possible for some genes to not activate or only partially activate, especially in kids. And if they do not produce enough brown eye pigments then their eye color will be blue or possibly a shade between blue and brown, green. This is more common in people with only one copy of the brown gene.

Our DNA is split in chromosomes which are paired. Each chromosome of a pair carry a version of each gene, one from your father, one from your mother. A gene carry the information on how to build a protein, so when your cells create a protein, it reads one of the version of the gene and follow its instructions.

Now, some genes are expressive and some are recessive. the expressive will be the one that will affect the phenotype (appearing parts) of your body while the recessive will probably won’t affect the phenotype.

For the blood types for example, if we oversimplify, there are 2 genes that define your blood type: one for the letter, one for the + (or the -). The letter have 3 variations: A when it tells to make the protein A, B for the protein B and O for “no protein”. there are 2 types of +/- genes: the + which creates the + protein, and the – that don’t create any protein. Since your body will read each gene in your DNA, and that you have 2 different genes, it will read 2 genes for the letter and 2 genes for the symbol.

If one gene is A and the other is A, it’s easy, it will produce A because that’s the only instruction there are, same with B. If you have 1 A gene and 1 B gene, it will produce both (so your bloodtype will be AB). If you have A and O, then your body will read the A and create A, and will read the O and will create nothing, so your bloodtype is A. Same with B and O. The only way to be O is to have 2 genes that tells your body to not create any A or B protein.

Now, for the eye colour, blue eye is recessive (IIRC, it’s just that it tells to not create melanin in your eyes) and the brown eye is expressive (it tells to create melanin in your eyes). So if you only have “brown eye” gene, your baby will at least have 1 “brown eye” gene. And since that gene is expressive, it will most likely means that your baby won’t have blue eyes. But that depends a lot on the gene you carry (how much expressive it is) and on the blue eye gene your partner have.

We generally model eye color as a simple trait that follows Mendelian Inheritance, aka brown is dominant so the only way you get blue eyes is if you have two recessive blue genes and no brown gene

In reality though, almost nothing is that simple. It looks like there are currently 15 genes believed to be associated with eye color and some exhibit dominance as you’d expect but others only have incomplete dominance so they may tweak the end value but not override it. You may carry mainly genes for brown eyes but some genes for blue eyes while he carries mainly blue eye genes and some brown that happened to have dominated. Their powered combined its possible to have a kid with only blue eye genes or a mix that presents as pretty much anything from blue to green to brown.

Very very few things are simple Medelian Inheritance, Mendel just got real lucky with his pea pods