Humans are born with 46 chromosomes that are composed of DNA, the determinants of genetic traits. These chromosomes are divided into 23 pairs, one of each chromosome contributed by each parent. Chromosomes provide the genes, or DNA sections, that determine each human’s physical traits. In a human, there are three different genes that are part of the two chromosomes that influence eye color.
A European monk named Gregor Mendel studied genetics and published his research in 1866. His theories were based on his study of plant genetics and were not recognized until 1900, six years after his death. His work has provided the basis for all genetic research. Much has been learned about genetics over the last century. Today, it is believed that several components, such as light wavelength and proteins, are part of the genetic tendencies for eye color inherited by every human.
Every child is born with bluish-colored eyes because their irises have not yet begun to develop the melanin, or the dark brown color necessary for eye color. In addition, melanin is created by exposure to the sun and so the full adult eye color is not usually achieved until one to three years of age.
The amount of melanin that will be produced in the iris of the eye is largely determined by genetics. Although there are at least nine possible combinations of genes, called genotypes, for any given eye color, for the sake of simplicity only blue and brown eye color genes will be discussed.
Each genotype is made up of four forms called alleles, two of which are inherited from the mother and two from the father. A copy of the bey 2 gene will be provided by each parent, as will a copy of the gey gene. The brown eye gene, called bey 2, is made up of a brown allele, or B, and a blue allele, or b. The other gene is called gey, and is made up of a green allele, or G, and a blue allele, or b. Eye color is dependent upon the combination of alleles inherited from parents.
The brown allele, B, is dominant over the G and b alleles of gey and bey 2. The b alleles are always recessive. If a person has only one copy of the dominant B allele, brown eyes will result. If a dominant copy of the G allele is present and the other alleles are recessive b alleles, then the result will be green eyes. If recessive b alleles are present, then blue eyes will result.
The shade of color inherited depends upon how much melanin is produced by an individual’s eyes. There are two types of melanin produced within the eyes—one results in green and hazel eyes, the other produces blue and brown eyes. Brown eyes have more pigment than other colors, and so absorb light before its rays scatter. This makes the eyes appear darker. Lighter color eyes, like blue, have very little pigment in the iris, so light gets through the iris and the short wave lengths of the lighter colors scatter the light rays. These short wave lengths make the eyes appear blue.
The determinants of eye color are much more complicated, however, as recessive genes from grandparents can influence the genetic traits inherited from parents.