Of all the things to imagine when daydreaming about your future baby, eye color may be one of the most common things for parents to envision. For some intended parent(s), eye color may be a big deciding factor in which egg donor to choose, especially for IP’s who are hoping for a specific eye color in their child.
Did you know that the inheritance of eye color is polygenic, meaning that eye color is determined by more than one gene? There are about 16 known genes responsible for eye color. So, in reality, predicting your child’s eye color is a little more complex than the simple inheritance pattern you may remember from high school!
Each person carries two copies of every gene, known as alleles. One allele is inherited from the mother (or egg donor) and one allele is inherited from the father (or sperm provider). For the purpose of this blog post, we will refer to the contributing egg donor and the sperm provider as the “genetic parents”.
The most common interaction between alleles is a dominant/recessive relationship. An allele of a gene is dominant when it overrides the other allele, which is recessive. When it comes to eye color, brown tends to be dominant over green or blue, and green tends to be dominant over blue.
Here is a simple example of eye color inheritance:
The allele for brown eyes (B) is dominant over the allele for blue eyes (b). So, if your child inherits one allele for brown eyes and one allele for blue eyes (Bb), their eyes will be brown. This would also be the case if they inherited two alleles for brown eyes (BB). If the child inherits both alleles for the recessive trait (bb), they will inherit blue eyes.
The above explanation provides an overly simplified understanding of how eye color is determined. However, as we discussed, eye color is influenced by multiple genes working together. The factors that distinguish eye color are complex and the eye color of your child cannot be definitively predicted. This is important to keep in mind when choosing an egg donor.
Homozygous means that the two alleles match (for instance, maybe both are for brown eyes). If someone is heterozygous, it means that the two alleles of the gene differ (for instance, one for brown and one for non-brown). It’s impossible to tell by simply looking at someone whether they have homozygous or heterozygous alleles for their eye color. Therefore, it is possible for two brown-eyed genetic parents to both be carriers of recessive traits for lighter eyes and pass that on to the child. If the child receives two recessive genes for lighter eyes, they will have lighter eyes.
There are many factors and several genes that influence eye color. There are two specific genes which play a major role in eye color: OCA2 and HERC2. These genes are responsible for maturing cells that produce melanin. Melanin determines how much pigment resides in the iris, or the colored part of the eye. A lot of melanin in the iris will produce brown or dark eyes and less melanin will produce lighter eye colors. Variations within these genes can reduce the amount of melanin that is produced. So, the resulting eye color depends on how the genes express themselves and the variability of how much protein is present in the cell, which contributes to how much (or little) melanin is produced. There are multiple possibilities for different shades of color depending on how all participating genes interact with each other.
The baby's basic genetic code is inherited from the genetic parents. However, it’s not a simple combination of dominant/recessive genes from the genetic parents, but rather, it’s dependent on the parents' genes mixing and matching in various ways. The mix of genes from both genetic parents blending together may produce somewhat random results.
We understand that for many intended parent(s), choosing an egg donor with specific shared characteristics to your family may be important. We hope this blog was helpful in explaining the complex factors that go into eye color and why the outcome isn’t always predictable. If you have any questions, our team is here to help you at every stage of your journey!