There are monogenic and complex traits within us. People are mostly taught how traits have two alleles and can be influenced with those genes. They provide a classic pattern with dominant and recessive. This is classified as monogenic traits. Yet, there are the traits that are influenced by several genes causing different variations, not following the strict dominant and recessive gene idea. Complex traits are when there are multiple genes involved and interact with environmental and behavioral factors. They also will influence the trait expressed to different levels within a person.
An example of a complex trait is the eye color in humans. Everyone has heard of the classic, brown eyes are a dominant gene, blue eyes are a recessive gene. But, have you wondered why there are also the eye colors of green or hazel or grey? If genetics followed the strict Mendelian inheritance, then there would only be people with blue or brown eyes. This is why eye color is a complex trait because it involves several genes in order to affirm an eye color of more than just the brown and blue. But, what is involved to make the different colors, what genes? To start off, eye color is the color of the iris of the eye. In the iris, there are two layers with the in between space called the stroma. In the stroma are different protein and white collagen fibers. In the back iris layer, it is mostly melanin, which is a dark pigment produced by cells, many people mostly refer to melanin as in the skin because it is what makes people tan. The part where eye color happens is with the front layer of the iris because the amount of melanin varies between people and with the eye color you have. Eye color will mostly be how the light is reflected upon the eye. If a person has brown eyes, then there is a lot of melanin and the light is being absorbed. If the person has less melanin than brown eyes, but more melanin than blue eyes, the color could be green or hazel. Lastly, if the person has little to no melanin, then the light actually reflects upon the collagen fibers causing the eyes to be blue.
Now on to the genetics on how the eye color is determined. To start, since brown is a dominant color, the first set of genes to determine eye color are the ones where whether or not you have brown eyes. If you have brown eye genes, then the second set of genes would not matter the color because they would be recessive to the brown color. But, if you don’t have brown eyes, then you go on to the second set of genes that determine whether you have blue or green eyes. With that set of alleles, the green color is dominant over the blue color. Eye color ties into complex traits because it involves multiple alleles all working together in order to express a person’s eye color.