"Polymorphism" is the name for when members of the same species exhibit different physical forms. The difference between male and female phenotype is an example of polymorphism in humans (and mammals/others). These differences are controlled by how chromosomes are inherited.

Suppose we had a species which has a second dimension of polymorphism - for simplicity, we'll call it being "red" or "blue". So an individual of this species can be a red male, a red female, a blue male, or a blue female.

Could the chromosomal inheritance of this species be structured to allow the following reproductive results? (How?)

  • The union of a red male and a red female will always produce a red female.
  • The union of a blue male and a blue female will always produce a blue female.
  • The union of a red male and a blue female will always produce a red male.
  • The union of a blue male and a red female will always produce a blue male.

(I'm hoping to keep the system as close to human chromosomal inheritance as possible; to avoid introducing completely new genetic mechanisms, etc.)


The simplest solution might be to have two chromosomes, a red one R and a blue one B. There are two versions, a "functional" version containing the "color" genes that is contributed by the male, and a "reduced" version contributed by the female that lacks those genes. Only the male version has the genes determining the color, so only the male's version impacts color. If someone has two chromosomes of the same color that is a female, while two different chromosomes makes a male. This is sort of analagous to the human X/Y chromosomes, where the Y version is reduced and lacks many genes.

The obvious problem is making sure only fathers contribute the functional version and only motgers contribute the reduced version. But there is already precedence for this in humans: only the mothers contribute mitochondria. The father's mitochondria are destroyed shortly after the sperm and egg fuse. So this species can have two organelles that carry genes. One such organelle carries the "functional" chromosome, and only the father's version of this survives. The other carries the "reduced" chromosome, and only the mother's version of this survives.

Strictly speaking these wouldn't have to be chromosomes, in fact such organelles probably would have simpler, reduced genomes the lack true chromosomes. The color can simply be determined by some genes of the male organelle, while the gender is determined by genes in both organelles.


Let's look at what you need (First letter for color, second for gender):

  • RM + RF = RF
  • BM + BF = BF

First two couples have the common denominator of having the same color.

  • RM + BF = RM
  • BM + RF = BM

While the latter two are exactly the opposite.
For this reason I'd suggest you make some system that resembles chromosomes X and Y. Those are responsible for weather we are born male or female. Have a red chromosome and a blue one. Now if a set of the same color exists (like in the first two pairs) - one of them is redundant, and it becomes a Y chormo. 2 Ys make a female, and the red chromosome will make the baby red.

The second group of pairs makes things a tad more complicated, I'd offer to link the XY chromosomes and the RB ones. Now you can say the Y chromosome (XY make a male) causes the pigment that comes with them from the parent to become dominant. This means the child will inherit the father's color.

This way you can satisfy your needs with two principles already in cation in all our bodies. One note though - with most things Biological, and definitely with genetics, there's no such thing as "always". you will always have mutations, and other 'hiccups'. Much like humans don't always have exactly XY or XX. Of course, it's your choice if you want to include something like that, after, things like that tend to be rare.

  • $\begingroup$ "2 Ys make a female" - shouldn't that be "2 Xs"? $\endgroup$ – G0BLiN Mar 14 '19 at 14:23

Absolutely it can be structured in that way. Genetics is very complex, and genes can be activated or inhibited in a staggering variety of ways. For example, enhancers are gene sequences that serve only to express (or rather cause the expression of) other sequences. There can even be multiple enhancers for a single primary sequence, each of them working under slightly difference circumstances.

In this case, it seems that, instead of being on totally separate chromosomes, both sets of sexual genes would be present in all individuals, but one set needs to be enhanced in order to be expressed, whereas the other will be expressed unless that set is enhanced: a sort of biological if/else block. Once you've established that, the enhancers - or other factors, like environment, but you've indicated you want a purely genetic reponse - that cause the non-default type to express can be as complicated as you like. In this case, if the default is to express female, it looks like the trigger to express male is having both "red" and "blue" codes; having just one is insufficient. Alternatively, the reverse could be true: default to male, express female if both codes line up.


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