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