Eli5 : Why do some genes (like skin color) mix and shade upon inheritance while some don’t (like eye color) ?


Eli5 : Why do some genes (like skin color) mix and shade upon inheritance while some don’t (like eye color) ?

In: 455

I always thought it was just one big genetic lottery!! Interested to see if there is logic to the science.

Skin color is controlled by a large number (I believe unknown) of genes and can average out. Eye color is [more narrowly controlled by fewer genes](https://medlineplus.gov/genetics/understanding/traits/eyecolor/). But like skin tone, the amount of melanin your body produces determines how dark your eyes are, and to have blue or green colors be visible, you need to be pretty damn light on melanin. But even of those who have brown eyes, there is [variety of just how dark it is.](https://www.google.com/search?q=variety+of+brown+eyes).

(IE, your premise is wrong, eye color does mix and shade)

Some traits are inherently dominant. If you have a gene to produce a chemical, it can sometimes dominate over its absence, because having any of it means you have the function. Blood type O is just the absence of A and B antigens, so once you have either an A gene or a B gene it doesn’t matter that your other gene is O. Your body is already producing the antigen, so you’re type A or B. Brown eyes are kind of the same, you won’t see the blue if there’s any brown there, so a second brown eyes gene doesn’t make a very visible difference.

With skin color, there are a lot of variables, and the range of color they produce lines up with the range of color we can see, which makes sense because its role is to regulate absorption of (almost) the same light we see with, over a wide range of environmental conditions.

If you had a trait to produce an important enzyme, and one gene out of two was defective, you might conceivably be fine as long as you could produce some. Or you might have a slight problem if you could only produce half as much. It depends on what you need it for.

So I have really blue eyes and my wife has a green/yellow/brown mix that we call hazel. Both our kids have her mix with my blue replacing her green. They look like marbles.

Eye colors do mix and shade.

My parents have blue and green eyes respectively. I have hazel eyes, which is a mixture of blues, greens and golds, and actually give the appearance of color-changing eyes because different colors stand out based on what color clothes I’m wearing.