• Types of iris are mostly defined by color.eye image by Pali A from Fotolia.com
  
  The iris is the colored part of the eye, which controls the quantity of light entering the ocular globe through the black-centered pupil. According to the optometrist Dr.Ted Montgomery, the color of the iris is a genetic characteristic "determined by the amount of pigment present in the iris structure." The color of iris is controlled by a gene on chromosome 15 and another gene on chromosome 19, but some researchers believe there are more genes involved, reports PubMed. Although patterns and color shades are unique to each person, there are three basic types of iris.
  
  

Blue

• Blue irises contain low quantities of melanin, the pigment linked to the brown color. According to Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen, the blue iris was originated from a genetic mutation, in a time when everybody had brown eyes, reported MSNBC Digital Network.
  
  

Green

• Green eyes are the results of medium pigmentation, originated by gene interactions along the human evolution. Apart from melanin, green eyes can have a brownish-yellow pigment called lipofuscin or lipochrome that also appears in amber irises, reports Dr. Larry Bickford O.D. at Eye Care Contacts. Green and blue eyes are common among Northern European populations. A research published by Science Direct found out that 89 percent of women and 87 percent of men in Iceland had blue or green eyes.
  
  

Brown

• Brown eyes have high levels of melanin, being very common among Africans and Asians. According to Dr. Larry Bickford O.D. at Eye Care Contacts, eye color is a polygenic trait and changes in color can happen in young children as a developmental response. The same phenomenon is not well-researched in adults.