Many babies are born with blue eyes, a common observation that often sparks curiosity. This initial eye color is not always permanent, and understanding why babies’ eyes often start blue and then change involves delving into the science of how eye color is determined.
The Appearance of Blue
The blue appearance of many infants’ eyes stems from the limited amount of melanin present in the iris at birth. Melanin is the pigment responsible for color in human skin, hair, and eyes. In newborns, the specialized cells called melanocytes, which produce melanin, have not yet fully activated or deposited sufficient pigment in the iris.
The eyes appear blue due to a phenomenon known as Rayleigh scattering. The iris contains collagen fibers in its stroma, the middle layer. When light enters the eye, these fibers scatter the shorter, blue wavelengths of light more efficiently than longer wavelengths. This scattering causes blue light to reflect back, making the iris appear blue. This mechanism is similar to why the sky appears blue.
Eye Color Evolution
A baby’s eye color often changes as melanocytes become more active after birth. Exposure to light stimulates these cells to produce and deposit more melanin in the iris. As melanin accumulates, it absorbs more light, which can shift the eye color from blue to green, hazel, or brown.
Most significant changes typically occur within the first 6 to 12 months. While many babies’ eye colors stabilize by their first birthday, some may continue subtle changes up to three years, especially for lighter eye colors. Dark brown eyes at birth are less likely to change significantly, as they already contain substantial melanin.
The Role of Genetics
While melanin development and light exposure influence eye color changes, the final eye color is primarily determined by genetics. Eye color is a complex trait influenced by multiple genes inherited from parents. These genes control the amount and type of melanin produced and stored in the iris.
The OCA2 and HERC2 genes, located on chromosome 15, are influential in determining eye color. The OCA2 gene produces a protein involved in melanin production, while HERC2 regulates OCA2’s activity. The combined effect of these and other genes dictates melanin production, ultimately determining the eye’s permanent hue.