<P> Blue eyes are rare in mammals; one example is the recently discovered marsupial, the blue - eyed spotted cuscus (Spilocuscus wilsoni). The trait is hitherto known only from a single primate other than humans--Sclater's lemur (Eulemur flavifrons) of Madagascar . While some cats and dogs have blue eyes, this is usually due to another mutation that is associated with deafness . But in cats alone, there are four identified gene mutations that produce blue eyes, some of which are associated with congenital neurological disorders . The mutation found in the Siamese cats is associated with strabismus (crossed eyes). The mutation found in blue - eyed solid white cats (where the coat color is caused by the gene for "epistatic white") is linked with deafness . However, there are phenotypically identical, but genotypically different, blue - eyed white cats (where the coat color is caused by the gene for white spotting) where the coat color is not strongly associated with deafness . In the blue - eyed Ojos Azules breed, there may be other neurological defects . Blue - eyed non-white cats of unknown genotype also occur at random in the cat population . </P> <P> In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed . </P> <P> Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present . Dark pigment of brown eyes is common in Europe, South Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa, Americas, etc. as well as parts of Eastern Europe and Southern Europe . The majority of people in the world overall have brown eyes to dark brown eyes . </P> <P> Light or medium - pigmented brown eyes can also be commonly found in South Europe, among the Americas, and parts of Central Asia (Middle East and South Asia). </P>

Percentage of the world that has brown eyes