If a child inherits two recessive genes, say one for blue eyes and one for green eyes, which trait will show?
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Blue is recessive to green. I think there are tiers of dominance, blue being the most recessive. IIRC
Genes per se are not dominant or recessive, alleles are. A given gene may come in many slightly different variations, and those are called alleles.
There’s nearly always one allele that’s more dominant than the other, because being a dominant/recessive allele is not an absolute status, it’s relative to what the other allele is. The same allele can behave as either recessive or dominant depending on what other allele it’s paired with.
There are cases in which neither of the two alleles really dominates over the other, that is known as codominace and results in both of them being expressed in the phenotype, for example the AB blood group is the result of the codominace between the allele A and the allele B of a gene called ABO.
Your specific example is flawed, since human eye color is not a mendelian trait despite what most people believe, meaning it doesn’t depend on just one gene.
Generally, there is no such thing as “two different recessive versions of a gene”. Recessive variants are usually a “broken”, “inactive” or “do nothing” versions. That’s why there cannot be two of them – there is only one way to do nothing. If there appears to be two recessive variants, it usually means that multiple genes are involved (as it is in your example with eye color).
There can be two dominant variants, however. If child inherits two different dominant variants, they usually interact with each other, creating kinda a mixture of both. An example is blood type gene: it has 3 variants – dominant A, dominant B and recessive 0. Those create 4 options:
* `00` gives O blood type
* `0A` or `A0` give A blood type
* `0B` or `B0` give B blood type
* `AB` or `BA` give AB blood type
Eye color is actually determined by multiple genes (as much as 15, each having their own dominant/recessive variants) – that’s why it does not follow simple rules. But, simplified: there is one gene, that determines “brown”(dom.)/”non-brown”(rec.), and all others that mix up into all other colors. The way they mix up is not trivial.
The following contains a bit of oversimplifying, but:
It depends. So, genes and DNA aren’t just “the code that says how you are”—they’re actually recipes to make all the proteins in your body. For example, your blood type genes tell your body “Here’s a recipe for the antigens on your red blood cells.” If you have two genes with Recipe A, you have little A antigens on your blood; if you have one A recipe and one B recipe, your body can read those recipes and make both of those antigens, and stick ‘em both on your red blood cells.
Now, eye color is a messy example—I did some reading (https://medlineplus.gov/genetics/understanding/traits/eyecolor/), and it turns out, multiple genes control it! Why? Because we have different recipes that affect it. I’m oversimplifying and tweaking the truth, but imagine recipes kind of like this:
-Do we have a recipe to make melanin, to pigment the iris darker?
-Do we have the recipe for the little protein friends who will move melanin into the iris?
-Do we have a recipe for a protein that says “Make more melanin recipe!” or “Stop making our melanin recipe!”
And so on.
There’s one more component: what’s recessive vs. dominant?
Well. Let’s say there’s a recessive gene that turns your whole body purple—if you have two “p genes”; you’re purple. If you’re PP or Pp, you’re not. Let’s pretend babies are purple in the womb, but most babies have a special protein, “P”, that removes the purple color before they’re born. If I have even one working P recipe, it can remove the purple, and I’m not born purple; but if I have pp from both parents, I didn’t get the purple remover, and I’m born purple. That gene p is recessive, because you need two ps to be purple. (A p recipe means our purple remover is broken or shaped wrong or something.)
Dominant genes are the opposite: babies being not purple is dominant, because one working P recipe removes the purple. So PP AND Pp babies are born their normal color, even if they only have one P recipe.
So, add these things together, and it’s super messy, and I can’t pretend I have an answer! If we could figure out all the processes that determine how an iris has the color it does, and we could tell which genes affected them, and if we knew which were dominant or recessive based on how they work, and we knew which specific “versions” the green- and blue-eyes parents had for all those genes… I don’t think we are 300% sure about eye color yet, at least in the article I read.
But Google says for blue + green it’s just 50-50 blue vs green, so what do I know 🤣 Hope this little rabbit hole makes sense and was somewhat interesting!
My parents were that, I have blue with green/yellow in the middle , my sister has green eyes.
Dominant and recessive are useful shortcuts, but fall apart quickly in real usage.
The more in depth read is that you have genes that have more or less impact. Brown eye gene does a lot, blue does quite little, green is in between. So with green you often end up with a mix with the other colour.
Some traits, especially eye, hair, an go like blend and aren’t binary. They are controlled by many genes. Eye color is oversimplified in biology classes. I can tell you my green-eyed mom and blue-eyed dad had two blue-eyed kids.
Most likely neither. Those are codes for two different recessive genes, and it requires the same recessive gene from both parents to be expressed in physical traits.
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