Eye colour is an inherited human trait. Parents pass on eye colour genes to their children, and eye colour results from the combination of those genes. Typically, brown-eyed parents have brown-eyed children, but not always. The most common eye colour is brown, followed by hazel, blue, green, violet and red. Eye colour may change naturally over time, and it can be changed artificially by the use of contact lenses.
The OCA2 gene has the most effect on individual eye colour. Variations in eye colour are controlled by three different single nucleotide polymorphisms (SNPs) that occur in the DNA sequence that occurs near the OCA2 gene. Different combinations of the three SNPs result in varying shades on the spectrum that ranges between blue and brown eyes. Darker eye colours are typically more dominant than lighter eye colours, but not in all cases.
Brown eye colour results from a dominant gene, and blue eye colour results from two recessive genes. Green eyes may result when variations occur within the OCA2 gene and in other related genes. Green eyes may possibly be triggered by the same gene that sometimes results in red hair.
Most infants have blue eyes at birth. If an infant develops melanin in his eyes, the eyes will darken and change colours. Some Caucasians continue to see subtle changes in their eye colour over the course of their lifetime. Some may have brown eyes that become hazel over time, or others may have hazel eyes that become just brown.
The most prevalent eye colour in the world is brown. Brown eyes may be various shades, from honey brown to very dark brown. Some Asians, Africans and Native Americans may have eyes so dark that they appear to be black.
Hazel eyes are a combination of brown and green colours. Hazel eyes most often occur in individuals who have European ancestry.
Blue eyes are less common than hazel and brown eyes and are usually found in individuals with European ancestry. Some blue eyes may be so light that they appear grey.
Green eyes are even less common than blue eyes. Green eyes are usually found in individuals with German or Slavic ancestry.
Other possible eye colours are violet and red, which sometimes occur in those individuals who suffer from albinism.
Changing eye colour is relatively easy with the use of coloured or tinted contact lenses. Coloured contacts can completely change the colour of the eyes, while tinted contacts can intensify the existing eye colour.