My son may ask this question to me one day because both of his parents have brown eyes and none of his grandparents have blue eyes. Well, the answer still lies in his genes.
Let me use the opportunity to explain what genes are. Imagine a long strip of random letters and while you are reading these letters in an order, you come across an actual sentence that makes sense. This strip of letters is your DNA while the meaningful sentence is a gene on it. So something like this:
Scientists use only 4 letters (A, T, G, C) representing 4 parts that build the DNA molecule. Yes, DNA is a molecule like the water molecule H2O, only more complex and much longer. Human DNA contains approximately 6 billion of these 4 letters in each cell. Meaningful arrangements of these 4 letters in some regions on the DNA are our genes.
There are about 20000-25000 genes scattered on the human DNA. Each one of us carries ‘versions’ of the same gene that differ from one another with just a single or few letters. Our genes not only determine how we look like but also contain the information on how our body works, from our eyes to our heart to our toes.
For each one of our genes, we inherit two copies from our parents. One from mum and one from dad. These two copies may be identical or distinct versions of the same gene. If distinct, one of these versions is sometimes superior (or dominant) over the other (the recessive one). The dominant version is then responsible for the outcome. Scientists used to think that eye colour was determined by a single gene with brown colour version dominating over the green or blue colour versions. Based on this, if both parents have blue eyes, they must have passed on to their children two copies of the recessive blue colour version, which means their children cannot have an eye colour other than blue.
Yet blue-eyed parents can have brown-eyed children. Why? As scientists would say, because it is more complicated than it seems.
Eye colour is determined by distinct versions of, mainly, two genes, generating multiple possible combinations. These two genes also need each other to function, which further complicates the process. Each one of us carries multiple eye colours in our genes, but only one appears in our eyes because some colours, like brown, are still dominant over the others. For example, a person with brown eyes can carry the blue eye version(s) of these two genes hidden behind the brown eye version(s). Then, if this person has children with another person who also carries the same blue eye version(s) secretly, their children could have blue eyes. Actually, such blue eye versions can stay hidden during generations of a family and then, suddenly appear in one of the children, like with my son.
Chances are low, but it is possible. So don’t be surprised if your child gets an eye colour totally different from the rest of the entire family. Biology has its own miracles.
If you would like to read more:
If you are interested in more science:
Sturm, R. A., and M. Larsson. 2009. Genetics of human iris colour and patterns. Pigment Cells and Melanoma Research 22: 544-562. Pubmed: 19619260 (Open access)
Sturm R.A., Duffy D.L., Zhao Z.Z., Leite F.P., Stark M.S., Hayward N.K., Martin N.G., Montgomery G.W. (2008). A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. Am J Hum Genet. 82(2):424-431. Pubmed: 18252222 (Open access)