Hazel eyes are a mix of brown, green, and gold tones within the iris. This eye color is distinctive because its appearance seems to shift, often looking more green in one light and more amber in another. Historically, eye color inheritance was taught as a simple trait where a single gene determined color, with brown being dominant over blue. This traditional Mendelian model is a vast oversimplification that fails to account for the spectrum of shades, particularly the complex coloration found in hazel eyes. The inheritance pattern for hazel, green, and other intermediate eye colors is much more complex, involving the interaction of multiple genes and varying levels of pigment.
The Core Answer: Is Hazel Dominant?
Hazel eye color is not categorized as a simple dominant or recessive trait in the way that brown and blue eyes were once defined. While the alleles that produce a high concentration of the brown pigment, melanin, are generally dominant over those that produce less pigment, hazel represents an intermediate outcome. The specific genetic combination that results in hazel eyes involves a moderate amount of melanin pigment, more than in blue or green eyes but significantly less than in dark brown eyes. Hazel is an expression of multiple genes working together to produce a mid-level amount and distribution of pigment. Predicting the appearance of hazel eyes is challenging because it is the result of inherited factors, rather than the simple presence or absence of a single dominant allele.
How Multiple Genes Determine Eye Color
The mechanism behind all eye colors, including hazel, is known as polygenic inheritance, controlled by the interaction of several genes. Researchers have identified as many as 16 genes that influence eye color, though two located on chromosome 15—OCA2 and HERC2—are considered the primary factors. These genes regulate the amount of the dark brown pigment, eumelanin, that is deposited in the iris. The OCA2 gene codes for a protein that regulates melanin production and storage, and the amount of functional OCA2 protein directly influences the overall pigment level. The HERC2 gene acts as a regulatory switch for OCA2, and this genetic interplay explains why predicting eye color is difficult and why hazel can appear in families without a history of the color.
The Unique Appearance of Hazel Eyes
The unique appearance of hazel eyes is due to both the concentration of melanin and the physical properties of light within the eye structure. Hazel eyes have a moderate density of melanin in the anterior stroma, the front layer of the iris, and this pigment is often distributed unevenly. The color is not derived from a distinct hazel pigment, as only brown-to-black melanin is present in the human eye.
The lighter, greenish, or golden hues are an optical effect caused by the way light interacts with the tissue and pigment. This phenomenon is similar to Rayleigh scattering, where shorter wavelengths of light, such as blue and green, are scattered and reflected by the translucent stroma. Since hazel eyes have less melanin than brown eyes, the combination of the scattered blue light with the moderate amount of brown pigment creates the perception of green and gold tones. The uneven layering of this pigment gives hazel its multi-toned, flecked appearance, often with a ring of brown or gold near the pupil and a greener color toward the outer edge.
Why Eye Color Can Change Over Time
The most common change in eye color occurs during infancy as a result of ongoing melanin production. Many infants are born with eyes that appear light blue or gray because their melanocytes, the pigment-producing cells in the iris, have not yet been stimulated by light. Exposure to ambient light after birth triggers the melanocytes to begin increasing melanin production and deposition in the iris. If the child’s genetic code directs the cells to produce a high amount of pigment, the eyes will darken to brown. If the genetic instruction is for moderate pigment, the eyes may shift from light blue or gray to green or hazel, with the final, stable eye color typically established around three years old.