As many as 16 genes are believed to have a bearing on the colour of our eyes.
The main two are OCA2 and HERC2. When a specific mutation occurs in the HERC2 gene it has an effect on the OCA2 gene, causing blue colouration of the eyes. This is because less melanin (brown pigment that is also responsible for tanning) is produced in the iris. If OCA2 is completely shut off by HERC2 then no melanin is produced in the eyes, a condition known as albinism.