If blue eyes are a recessive gene and green eyes are a dominant gene, why are there more people with blue eyes?
In: 151
Green eyes are just more rare.
Think of it this way. Singing really loud at 2 am is dominant to quiet slumber. But why is it that most people get a good nights sleep? Because there is very little singing really loud going on at 2 am.
There just aren’t a lot of green eyes out there to be available to dominate blue eyes.
[deleted]
Eye genetics are much more complicated than dominant and recessive. While there are two main genes, there are other genetic factors. Also, the fact that there are two genes alone makes the situation more complex. There even appear to be two specific genes to determine blue vs green.
Brown is more dominant over green and blue, green is dominant over blue, and blue is recessive. Recessive genes require two of the recessive genes in order to exhibit the trait (bb). That means a blue-eyed parent is 100% guaranteed to pass on the recessive gene for blue eyes. Whereas a parent exhibiting a dominate trait can have many different combinations (BB Bb BG) for brown and (GG Gb) for green. Since each parent can only pass down 1 gene each to their child, you can see a trend. A brown-eyed parent has a 66% chance to pass on the brown gene, a 16% chance blue, and 16% green. A green eyed parent has a 75% chance to pass on the green gene and a 25% chance to pass on the blue. But, again, a blue-eyed parent has a 100% chance to pass on the blue gene because the blue gene is all they have. So, really, because green is less dominant than brown AND harder to pass on than blue, you find it less commonly. What you’d have to do to prove it is make a chart and match up all the possibilities to see what the results would be. Thankfully, this has already been done, and I can link you a nice visual of the results.
https://www.johnoconnor.co.nz/wp-content/uploads/2018/08/eye-colour-gene-chart.png
TL;DR Brown genes dominate green and blue genes, green genes dominate blue genes, and blue genes are easier to pass on than green genes. When you consider all 3 possibilities, green is mathematically less likely.
Put very simply, dominance doesn’t equal to prevalence. I don’t remember alot from high-school biology but this was one of the few facts that stuck with me only cuz it boggled my mind that the six finger gene is dominant to the five finger gene too.
[removed]
Eye colors are a bit more complicated than just a single gene that decides it, but even in this simplification it can be explained as such:
A gene being dominant doesn’t make it more likely to be passed on, and some genes are simply more prevalent (less rare) than others. This can lead to the situation where bb is more common than any combination of genes that makes your green eyes express.
Recessive doesn’t mean rare and it doesn’t mean bad. The O blood type is recessive but it’s the most common blood type in the western world.
[removed]
Worldwide there are more people with brown eyes than blue eyes; this is true in the United States too. https://www.healthline.com/health/eye-health/eye-color-percentages
The commonness of an allele isn’t determined by it’s dominant or recessive nature. The commonness of an allele depends on how much that allele promotes the survival and reproduction of the individual with the allele in a specific environment. Here brown eyes, associated with melanin production, is the result of selection for survival in environments with intense sunlight in the absence of clothing to protect skin and its cells from UV light. UV light can cause mutation in skin cells, one outcome of mutation is the formation skin cancers.
The frequency of skin cancer does depend on racial ancestry with caucasians having the least skin melanin and the most skin cancer. Similarly individuals having negro ancestry have more skin melanin and the lowest incidence of skin cancer. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2757062/
If we accept the African evolution of humans, and the out of Africa migration of humans, then our ancestors were brown eyed. It is as humans migrated into latitudes with less sun intensity that there was less selective pressure for melanin production, that non-brown eye colors become apparent.
Latest Answers