The colored part of the human eye, surrounding the black
pupil, is called the iris, a word deriving from the personification
of the rainbow in Greek mythology The color of the iris
is usually brown, blue, green or hazel
The color of the iris is provided by a pigment called melanin,
which is also present in the body’s hair and skin, although
this is of a slightly different type The melanin in the
iris is yellow-brown to dark brown in color, and the amount
of melanin in a person’s irises will determine their eye color—
for instance, a person with a high level of melanin will
have brown eyes, while someone with less melanin in their
irises will have blue eyes People whose skin and hair are
dark tend to have more melanin and darker eyes than people
whose skin and hair are fair
It was originally thought that eye color was determined
by one specific gene, but it is now believed that a number
of genes are involved The genes that cause brown eyes are
dominant, so, if one parent has brown eyes and the other parent has blue eyes, the child is more likely (though not
certain) to have brown eyes It is, however, possible for a
brown-eyed parent to produce a child with blue eyes if the
parent’s eye color is the result of carrying recessive blue-eyed
genes as well as the dominant brown-eyed genes In addition,
some blue-eyed babies’ eyes darken as their levels of
melanin increase with age.
It is possible, although rare, for a person to have different
levels of melanin in each eye, a condition called heterochromia
iridium, which results in each eye being a different
color This condition can be inherited or can result from an
alteration of one of the relevant genes Alexander the Great
had this condition A similar condition, heterochromia
iridis, manifests itself as a variety of colors within one eye.
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