Both you and your partner have brown eyes, so your baby will have brown eyes, yes? Or, one of you has brown and the other has blue, so there's a 50/50 chance it could go either way, right? Well, it's not really that simple.
"Things have become a whole lot more complex than the simple recessive/dominant model that people used to talk about," Associate Professor Robyn Jamieson from the Children's Medical Research Institute at the University of Sydney told The Huffington Post Australia.
And interestingly, brown eyes are not the most common which is often thought the case.
"It depends upon what population you are speaking about," Professor Richard Sturm from the Dermatology Research Centre at the The University of Queensland told HuffPost Australia.
"In Australians of European ancestry, the percentage of eye colours are 45 percent blue-grey, 30 percent green-hazel and 25 percent brown. If you're considering non-European ancestry it is the almost completely brown eye colour."
But back to what peepers your bub might have.
"Eye colour has generally been considered as a good text book example to help explain the concept of a recessive trait in that brown eye colour is generally dominant to blue eye colour. However, this ignores the complexity of 'non-blue' eye colour," Sturm said.
"The range of eye colour seen in our population reflects the range of skin colour, with all hues in between brown and blue, however we often talk about green-hazel as the 'intermediate' colour. Eye colour is therefore not a simple recessive trait, in that it is often explained there is one gene with a dominant and recessive form, it is actually dependent upon a number of genes -- a polygenic trait."
Say what? Basically, when you add green-hazel into the mix, things get complicated. And when you consider that genes are passed down through countless generations with all sorts of variables, its very hard to accurately guess.
"Having said this, about 75 percent of eye colour variation can be explained by one gene. This is known as OCA2, which has a major dominant form for brown eye colour and recessive for blue eye colour. This gives the ability to predict if someone has light or dark eye colour but additional genes will also influence the overall colour," Sturm said.
"As eye colour is often taught or spoken about as a simple recessive trait, it is usually presumed that two blue eyed parents will give a blue eyed child. In contrast, two parents with brown eye colour will usually have children with brown eye colour, but that blue is also possible. It is not an unreasonable assumption and it may generally turn out to be the case, but this does not mean other inheritance patterns are not possible -- indeed they do happen, just not commonly."
So if you both have blue eyes, you'll probably have a blue eyed baby. If you both have brown, expect a brown eyes baby, or maybe blue. If you add green to the mix, it's a lucky dip. And so are the joyous mysteries of procreation.