Iris

IRIS… AND EYE COLOR…

Do you know you have a unique eye color differing from everybody else’s in the world? This happens because, even if presenting blue eyes just like your brother, each individual hosts a color with distinct forms of emerging and manifesting.

Human eye color depends on several genes and, unlike popular belief, it is not a simple mixing of parent’s colors. It is also frequent to speak about brown eyes being “dominant” and blue eyes being “recessive”. This is actually not that simple to predict. Transmission of genetic characteristics is a very complex process and so it is possible for a couple with blue eyes to have, for instance, a child with brown eyes.

Color depends on quantity and distribution of a brown pigment in iris called melanin, which is also the pigment responsible for skin color. In a simple way we can affirm that brown eyes have more melanin than blue eyes and there are a lot of color tones between them. Melanin quantity in iris and its distribution is unique in each individual and a decisive factor for genetic identity.

Iris is a structure containing muscles and other cell types. It is anatomically divided in two layers: pigment epithelium (upper or “back” layer), which has a brown pigment for almost everybody – even for blue-eyed people –, and stroma (“front” layer) where melanin quantity imposes final color pattern for each individual. This means that blue-eyed people have no special blue pigment; they simply do not have melanin (brown pigment) in iris stroma, which causes a greater light dispersion once iris absorbs light from longer wavelengths and reflects shorter ones – just like water and sky get their blue tone! This is why we can perceive eye tonal variants according to lightning conditions.

Most babies are born with bright-greyish eyes (relative melanin absence) that may become darker throughout the first months if melanin develops. If both parents have brown eyes, it is more likely to children have brown eyes too. Darker eye color tends to be dominant, hence brown usually “wins” green, and green usually “wins” blue. This does not mean that a child born from parents with both brown and blue eyes will always have brown eyes, as previously mentioned.

Brown is the most common eye color in the world. Nevertheless, and as previously mentioned, it is not inherited from a single gene – in fact, recent studies show that eye color may depend on up to 16 different genes… On the other hand, every blue-eyed people have a specific genetic trait, which tell us they share a common ancestral past! Green eyes are more rare, existing only in 2% to 8% of world’s population. Green eyes emerge not only due to melanin quantity but also because of the way light is scattered when finding the iris.

Besides this, as we all know, and recalling Francisco José Galopim de Carvalho’s song… brown eyes are loyal… blue eyes are jealousy… green eyes are treason… black eyes are complaint…

 

Despite the use of such associations in very heated arguments, there is still no scientific evidence about them!! 🙂

 

Sources: American Academy of Ophthalmology – https://www.aao.org/eye-health/tips-prevention/why-are-brown-eyes-most-common;
American Association for Pediatric Ophthalmology and Strabismus – https://aapos.org/patients/eye-terms

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