Eye colour depends on your genes

I've got a question for you about eye colour. My extended family, going back generations, all have dark brown eyes. On my husband's side of the family tree, there is only one person with hazel eyes and nobody with blue eyes. Yet our son was born with bright blue eyes, which was a big surprise for us (no postman comments, please). Can you explain how this happens?
The genetics of eye colour are fascinating, complex and only partially understood. We take it for granted that blue is a recessive gene, buried under the dominant brown genes, but it's not quite that simple.
Eye colour is determined by dozens of genes, with the six most important genes interacting to create 90 per cent of the colour variations. But genes can show incomplete penetrance and variable expressivity, which means that genes (even dominant ones) can be passed from generation to generation without actually being expressed. This is one of the ways in which genes can "skip generations" and cause surprises.
And although we traditionally associate blue eyes with Scandinavian ancestry, the genes are actually spread fairly widely. They are thought to have arisen in Eastern Europe during the Neolithic period and spread from there to Northern Europe, the Middle East, Central Asia and even parts of India. Odds are good that, as a New Zealander, you've got some of these genes in your genetic makeup, too.
It is common for Caucasian babies to be born with eyes that appear light coloured and blue or grey-tinged. This happens because their irises are not yet fully pigmented.
It takes a year, sometimes two, for their eyes to become fully pigmented and during this time those baby blues can turn a darker shade of blue, hazel, or even brown.
Gary Payinda MD is an emergency medicine consultant in Whangarei.
Do you have a science, health topic or question you'd like addressed? Email: drpayinda@gmail.com
(This column provides general information and is not a substitute for the medical advice of your personal doctor.)