# How can I determine gene expression in cases of multiple co-dominant genes? [closed]

On a planet far, far away, there exists a human species with many appearances and abilities. There is a form of energy that flows through air, water, earth and plants (but not humans or animals) which I will call "aether" for now. Each human has a core. The core determines the abilities (and to some extent the appearance) of each person. Some core types allow passive tapping of aether, some active, and one only allows viewing of aether.

There are a minimum of 8 core types with 11 total possible types.

The possible genes/alleles involved are:

1. S/s
1. B/b
1. M/m
1. F/f
• 5-7. H/h (H is sub-divided into HR/Hr and HM/Hm)
• 8-9. E/e (E produces one result, ee produces another)
1. G/g
1. "X" (undetermined since I haven't decided by what mechanism this type appears)

Hopefully to make it clearer:

1. S = shapeshifters [passive aether use].
• 2-3. B and M both have claws and a tail, but B = lightweight (think wallcrawling/speedy) and M = tank [passive aether use].
1. F = wings [passive aether use].
• 5-7. H (all types) = healers of different types [undetermined on aether use, passive?].
• 8-9. E (both types) = aether users (E = ignition type, ee = open type) [active aether use].
1. G = aether users (external type) [active aether use].
1. X = aether seer [non-aether user] - I haven't determined this one yet.

So if a person has ss bb mm ff hh ee Gg then they should express type G (obvious problem here with the recessives though).

At the moment, I have it set up so double dominants override single dominants and single dominants override recessives. Where that falls down is threefold:

• I don't have any viable solution for what happens if someone has 2 or more double dominants
• at least 2 types currently depend on recessive genes which would make them ridiculously rare
• there is no explanation for what happens to a person with all recessives

What is a valid way to make this work genetically?

I don't want type inheritance to be random, hence why I made it genetic, but there has to be the option for children to have a type different to their parents, and that to be common enough that nobody blinks at it. I will consider limited environmental factors i.e. if the environment is hot then someone with both MM and GG will be M type, for instance. I can add further info on the types if that will clarify.

• ee/Open type aether users appear at a rate of 1 to every 2-4 EE/Ee ignition type users (which is why I put it as recessive to ignition type).
• Type X is extremely rare (as in only a few in the country rare, and not sure if it's inheritable).
• Each country will have a mix of types although some will have predominantly one type depending on how the environment favours it.

(This is connected to my other question here: How would I go about establishing phenotype in the case of co-dominance where mixed phenotype is not an option? which is specifically about the HR/HM gene)

• Vote to close. Needs clarity. What the heck does things like S/s and G/g mean? Commented Apr 8, 2023 at 19:55
• Uh...those are the alleles? I'll amend the question. Commented Apr 8, 2023 at 19:58
• Genes are complicated. They don't really have 1 way of doing things. Commented Apr 8, 2023 at 19:59
• @Solitaire I still have no clue what you are talking about. It involves gene pairs and superpowers sure. But "So if a person has ss bb mm ff hh ee Gg then they should express type G" is Greek to me. Commented Apr 8, 2023 at 21:58
• @Daron In other words, if all genes are recessive excepted one which is a dominant "G", the person has the ability of using aether actively (what the gene G does). Though... Now that you say it, it could be indeed clearer. It's personally hard for me to understand the different types of aether users. Commented Apr 8, 2023 at 22:13

"How can I determine gene expression in cases of multiple co-dominant genes?" You don't.

Not at the current level of understanding of the machinery.

In the case of real-world biology, you puzzle it out from observation; in the case of fictional genetics, you postulate it and invent a plausible just-so story.

The problem is that genes do not encode phenotypic traits. What they encode is proteins. One gene encodes one protein¹. It is the rest of the machinery which processes the proteins and, if all goes well, expresses a phenotypic trait. Unfortunately we the humanity are still very far from having a good understanding of the rest of the machinery.

¹) Not completely true. One gene encodes either one protein (normal gene) or one non-coding RNA molecule (so-called RNA gene). Those non-coding RNA molecules are unfortunately part of the "rest of the machinery" which is still poorly understood.

If you are a programmer, maybe the following analogy will be useful: what we have at this point are only a disassembler and an assembler. We have only the vaguest idea of the hardware architecture and of the operating system. We know that 66 01 d8 means add ax, bx, that cd 21 means int 21h, and that e4 2d means in al, 2Dh. In some cases, such as add ax, bx, we know what the instruction means. But in cases such as int 21h or in al, 2Dh, while we know in principle their meaning, we have no idea why they would be used. What's at int 21h? What peripheral is addressed by port 45? Only by understanding the hardware architecture and the operating system can such questions be answered.

In some rare case, the one protein encoded by one gene is directly visible in the phenotype. Such simple Mendelian cases are very rare. The vast majority of genes are pleiotropic, meaning that they influence not one but several, possibly even many, phenotypic traits; and most phenotypic traits are polygenic, meaning that several different genes must align just right for the phenotypic trait to appear.

So, what do you do?

If this was a real biological problem, you would look at the phenotype and at the genotype and try to find a relationship. For practical examples of polygenic traits of which the genetic determination is understood, look for example at the genetics of horse colors, or at the genetics of cat colors, or at the genetics of human eye colors. Human hair color is not a good example, because we still don't have a good understanding of its genetic determination.

But this is not the real world. It is made-up genetics. Therefore you can assign whatever meaning you want to your genes. And, most importantly, you can invent:

• Regulator genes which control the expression of the genes of interest.
• Switch genes, which enable or disable the expression of other genes.
• Other non-genetic control mechanisms, such as DNA methylation and epigenetic methylation in humans, which add spice and variety to the attempt to determine gene expression in cases of multiple co-dominant genes...

Hierarchy of Powers

I have it set up so double dominants override single dominants and single dominants override recessives. Where that falls down is threefold:

Look at the pairs. If there is a double dominant pair then the rightmost DD pair is your superpowers. If there is no double dominant skip to the next step.

For example a person with (ss BB mm ff HH ee gg) has two DD pairs. The rightmost pair is HH so they manifest healing powers.

For another example a person with (SS Bb mm GG Hh EE gg) has three DD pairs and some SD pairs. Ignore the SD pairs. The rightmost pair is EE so they manifest Electrical powers.

The next step

If there are no DD pairs then the rightmost SD pair is your superpowers.

For another example a person with (Ss Bb mm Gg Hh ee Gg) has no DD pairs and five SD pairs.The rightmost pair is Gg so they manifest Gravy powers.

# Gene expression through environment

Gene expression isn't fully deterministic. Though many traits are nearly fully deterministic, like eye colour, many do not. Gene expression can change depending on the environment.

Let's take grain for example. Though they do not have an identical gene make-up, we can rely on the law of big numbers. The grain in the middle of the field tends to have shorter roots and taller stalks. The grain on the edge of the field tend to have longer roots and shorter stalks. Why would grain with more or less the same genetic makeup be so physically different? This is where environmental gene expression comes into play.

The reason for the difference is simple. The grain on the edge is buffeted more often and by stronger wind. To prevent being uprooted it will grow less high for less wind, as well as rooted more deeply for more stability. The next plant is a bit more sheltered, as is the next, so most in the field get less wind. They grow taller for many evolutionary reasons. For each their environments trigger some genes, while others lay dormant.

The same applies to humans in many ways. The ability to resist hunger, some diseases or disorders, or some positive traits. The genes can be triggered. Some can be put back in the box easily, while others can still be found generations later.

So even with all recessive traits the order does not have to be deterministic. It can be as easy as exposure (or failure to be exposed to) some environment or life style. This gives great liberty in your story, while still rooted in reality.