Eye colour genetics. Computer artwork depicting the genetics of eye colour. The colour of the iris of the eye depends on the amount of melanin contained within its cells. If a lot of melanin is present, the iris is brown in colour. However, if little melanin is present, the iris appears blue due to scattering of light by collagen fibres. At least three genes are known to influence eye colour in humans by controlling the amount of melanin present. Genes are located on chromosomes (centre left). Different versions of genes are known as alleles. The allele for brown eyes is dominant over the allele for blue eyes.