Making Brown Eyes Blue

I’ve noticed in my discussions with people that simple concepts of heredity and genetics are poorly understood. Here are some basics. Any given trait can be dominant or recessive. Dominant genes will physically manifest any time they are present, including when they are paired with a recessive gene. A recessive gene will only be physically manifested if it is paired with another recessive gene. Additionally, there are three basic scenarios. Each parent can contribute a dominant gene, each can contribute a recessive gene, or one can contribute a dominant gene and the other can contribute a recessive gene. Let’s look at eye color as a simple example. In this case, the brown eye gene is dominant and the blue eye gene is recessive.

Each person gets an eye color gene from their mother and one from their father. If mom has two brown eye genes (one from her mom and dad) and dad has two brown eye genes (one from his mom and dad), then all their kids will have brown eyes. If mom has two brown genes and dad has two blue eye genes, then all their kids will have brown eyes since the dominant gene will always be present. If mom has a brown eye gene and a blue eye gene and dad has two brown eye genes, all their kids will again have brown eyes. If mom has a brown eye gene and a blue eye gene, and dad has a brown eye gene and a blue eye gene, their kids will have a 25% chance of having blue eyes. If mom has two blue eye genes and dad has a brown and a blue eye gene, their kids now have a 50% chance of having blue eyes. Lastly, if mom has two blue eye genes and dad has two blue eye genes, all their kids will have blues eyes.

This is how simple genetics work. However, it is actually possible that two blue eyed parents can have a brown eyed kid because eye color is polygenic meaning that several genes interact to determine eye color. Still, the likelihood of two blue eyed parents having a brown eyed child is very small and so in most cases the basic model still applies.

Hopefully this clears up some misunderstandings about basic genetics. When you hear people say something like “How did two right handed parents have a left handed kid?” (or whatever trait they are talking about), you will know that each of the kid’s parents had both the dominant and recessive genes for the trait and they each contributed the recessive gene. Now, that’s an idea worth passing on.