Dear Reader,

At birth, babies are usually born with blue colored eyes, regardless of the eye colors of their parents. At this early stage in life, newborns have not started to make a dark brown pigment called melanin in the irises of their eyes. As babies get older, their eye color may change when they start producing melanin in their irises. Melanin is also produced by the skin when it?s exposed to UV rays to protect against their harmful effects. This pigment also causes a suntan to develop by darkening skin color.

Similar to skin tone, the darker the eye color, the more melanin is present. Iris color falls along a spectrum from blue, which equates to a little melanin, to dark brown or even black, which equates to a lot of melanin. Gray, green, hazel, and lighter brown colored eyes fall in between these two ranges.

Eye color is a physical trait that is determined by the pairing of genes from both parents. It used to be thought that a single gene pair following dominant and recessive inheritance patterns was responsible for this trait. Now it is known to be much more complex, involving at least three gene pairs. Geneticists have focused on two of the three gene pairs to help clarify the inheritance of eye color: EYCL₁ (the gey gene) and EYCL₃ (the bey2 gene).

Genes come in different forms, called alleles. The gey gene has two alleles, green and blue; the bey2 gene has two alleles, brown and blue. The brown allele is always dominant over the green and blue alleles, the green allele is always dominant over the blue allele, and the blue allele is always recessive.

It is possible for two brown-eyed parents to have a child with a different eye color than theirs. For two brown-eyed parents to have a blue-eyed child, for example, the mother and father would need to pass on a pair of blue alleles each to their offspring. If this child were to get one green allele in this mix instead, s/he would have green eyes; however, if a brown allele is present, regardless of what the other three alleles are, the child would have brown eyes.

But wait, you say: this two gene model cannot explain the inheritance of gray or hazel colored eyes, nor can it explain how two blue-eyed parents can have brown-eyed progeny. It appears that as of yet to be determined modifier genes, other eye color genes, mutations, and/or possibly additional factors are involved in eye color inheritance.

Alice