The Complexity of Eye Color Inheritance

Eye color is a complex trait that is determined by several genes, and not just one as was previously thought. The brown eye allele is the most dominant, followed by the green eye allele, and then the blue eye allele, which is always recessive. Brown and green alleles will always outperform blue alleles, with brown being the most dominant. If both parents have a blue allele, the child is likely to have blue eyes.

However, if one parent has green eyes and the other has blue eyes, your child most likely has green eyes, since green dominates blue. Genes come in pairs and each gene in a pair may differ slightly. One gene can be dominant and override the other gene, which is recessive. A recessive gene only works if both genes in a pair are recessive.

Although this is not as simple as shown here, blue is the recessive gene and brown is the dominant gene. The Punnett square (known as Mendelian inheritance) illustrates how different combinations of dominant alleles and recessive alleles are observed. An allele is a version of a gene (the eye color gene may consist of blue, brown, green, gray, and hazel alleles). For example, brown eyes are the dominant gene for eye color and blue eyes are recessive, so when genes from brown and blue eyes combine in offspring, there's a 75% chance that offspring will have brown eyes.

That's why most people in the world have brown eyes. A person with brown eyes can carry a blue allele and a brown allele, so a brown-eyed mother and a blue-eyed father could give birth to a blue-eyed child. At least one polymorphism in this area of the HERC2 gene has been shown to reduce OCA2 expression and decrease protein P production, resulting in less melanin in the iris and lighter eyes. A lower amount of protein P means there is less melanin in the iris, which causes blue rather than brown eyes in people with a polymorphism in this gene.

In a second study, a group of 443 young adults of both sexes and different eye colors were asked to report on the eye color of their romantic partners. Following the logic of yellow and green peas and making a Punnett square, it should be impossible for two blue-eyed parents to have a brown-eyed child, but it would be possible for brown-eyed parents to have a blue-eyed child if both parents were a genetic mix of a blue-eyed gene and a brown-eyed gene. Ocular albinism is characterized by a very low pigmentation of the iris, which causes very clear eyes and significant vision problems. An important point is that even if brown eyes are dominant, and if there is no group of brown-eyed people to reproduce with, blue eyes will continue to persist. If both parents had a blue-eyed allele and a brown-eyed allele, their child would have a 25% chance of having blue eyes.

Both blue-eyed and brown-eyed women showed no difference in their preferences for male models of both eye colors. Eye color inheritance is more complex than originally suspected because several genes are involved. Researchers used to think that eye color was determined by a single gene and followed a simple inheritance pattern in which brown eyes were dominant over blue eyes. However, this is not always true as there are many factors that can influence eye color inheritance.

Marlene Manwaring
Marlene Manwaring

Infuriatingly humble twitter geek. Freelance internet practitioner. Hipster-friendly food maven. Subtly charming beer buff. Award-winning zombie geek.