Eye colour is determined by multiple genes, though the actual number of genes involved remains unknown.
The most simplistic explanation of eye colour inheritance, using just blue and brown eyes, is as follows. You have one eye colour gene from each parent. If either or both of these genes are “brown,” you will have brown eyes. If neither is “brown,” you will have blue eyes. Remember, each of your parents has two genes from their parents as well, so it is possible for two brown-eyed parents to have a blue-eyed child. How? If each of the parents has a recessive “blue” gene that they transfer to their child. However, if both parents have blue eyes, it is impossible for them to have a brown-eyed child, as neither of them has a dominant “brown” gene.
This is obviously oversimplified, as more than one gene controls eye colour, which allows for various degrees of pigmentation.
Beyond its initial expression, eye colour can also change throughout life, particularly during puberty, early childhood, pregnancy, and occasionally major trauma. Many children born without pigment – with blue eyes – develop pigment later on as cells within the iris produce melanin.
Mutations can also cause unusual eye colours, such as black, red, or violet. In fact, scientists believe that blue eyes originate from a mutation that occurred between 6,000 and 10,000 years ago. This mutation reduces the production of melanin in the iris, which dilutes the standard brown colour to blue.
In recent generations, the presence of blue eyes in the United States in particular has dropped significantly. A 2002 Loyola University study revealed that 57.4% of Caucasians born in the United States between 1899 and 1905 had blue eyes, compared to 33.8% of those born between 1936 and 1951.* As of 2006, only 16.6% of Americans had blue eyes.** This can largely be attributed to the increasing diversity in America, particularly as increasing numbers of Americans marry outside their ethnic group.