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Nature does some pretty interesting things with gender:

This is a female trilobite beetle.

enter image description here

Scientists have observed females extensively, yet for decades, scientists could not identify the male triolobite beetle. Of course there had to be one, as the females cannot reproduce asexually - but the males had never been observed until recently because they look completely different.

This is what a male looks like.

enter image description here

While nature can produce some extreme circumstances like these, I'm interested in pushing it farther. In a relatively complex, but not necessarily intelligent, two-gendered species,

What can be the greatest possible genetic difference between males and females, and

What are 'cheaty' ways to make them even more diverse?

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    $\begingroup$ This article has some cool real-world differences: en.wikipedia.org/wiki/Sexual_dimorphism#Insects $\endgroup$
    – CHEESE
    Dec 6, 2016 at 13:11
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    $\begingroup$ Darn. As usual the female looks much better than the male. $\endgroup$
    – Alexander von Wernherr
    Dec 6, 2016 at 13:41
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    $\begingroup$ You can look at angler fish the males is little more than a testes once it attaches itself parasitically the the female en.wikipedia.org/wiki/Anglerfish $\endgroup$
    – John
    Dec 6, 2016 at 15:25
  • $\begingroup$ @AlexandervonWernherr as usual huh? goto the link CHEESE mentioned... $\endgroup$
    – Mukul Kumar
    Dec 6, 2016 at 16:47

4 Answers 4

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I also think you could have 100% genetic diversity, but not as Philip suggests, since you are looking for the male and female to be of the same species.

Jason K gave you a good answer, but I would like to expand it even further.

Now, the example I'm about to give you is just theoretical. There is not, to my knowledge, anything like it on the real world.

Humans have 46 chromosomes on each cell, which constitute the totality of their genetic code. There is 1 pair of sexual chromosomes (XX in women and XY in men) and 22 pairs of other chromosomes (these are named autosomal and code everything else in the body).

Let's assume a species with the same genetic configuration, but in which male and female are so diferent, they look like diferent species.

First, you must know that, although we have a genetic code, not all genes are expressed all the time. Some are up-regulated and some are down-regulated. Which means, some genes are activated and some genes are inactivated. A liver cell will express a diferent set of genes than a brain cell, even though the genetic code in both is the same.

Now, imagine that each member of that species has

  • 2 sexual chromosomes, that determine the individual's sex (let's call them S chromosomes)
  • 22 chromosomes that contain genes for a male member of the species (let's call them M chromosomes)
  • 22 chromosomes that contain genes for a female member of the species (let's call them F chromosomes)

The S chromosomes would determine if the individual would be a male or a female. How? The S chromosomes of a male would "silence" the F chromosomes. And the S chromosomes of a female would "silence" the M chromosomes.

So, a male would only express the genes from the 22 M chromosomes and a female would only express the genes from the 22 F chromosomes.

This means that the content of the M and F chromosomes could be completely diferent. The only thing that would have to remain equal for both sexes would be characteristics that would allow genital and environmental compatibility, so that males and females could reproduce.

What about the genetics of reproduction?

In humans, the gametes contain half of the totality of the genetic code, ie 23 chromosomes (being 1 sexual and the other 22 autosomal).

When the 23 chromosomes from a sperm combine with the 23 chromosomes of the ovum, we get 46 chromosomes again: this is a new individual.

In your fictitious species, you would have to work this out a little further, lest a male individual gets a majority of F chromosomes (and vice-versa), which would be incompatible with life.

In the production of the gametes of your species, you would need them to contain:

  • 1 S chromosome
  • 11 autosomal M chromosomes
  • 11 autosomal F chromosomes

When the gametes combined, this would render 46 chromosomes again:

  • 1 S chromosome from the father + 11 M chromosomes from the father + 11 F chromosomes from the father
  • 1 S chromosome from the mother + 11 M chromosomes from the mother + 11 F chromosomes from the mother

In this way, you would ensure genetic diversity because a female daughter wouldn't get all her F chromosomes from her mother, but rather half from her mother and half from her father (yes, the father would transmit half of his own inactivated F chromosomes). You would also ensure that every member of the species would have the set of chromosomes required for their assigned sex.

So, it is theoretically possible to have 100% genetic diversity, since the genes expressed in both sexes would be completely diferent. Even though, of course, there would not be so much genetic diversity if you took the genetic code as a whole.

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Cheaty answer: 100% diversity, because males and females are actually two completely different species, but have a symbiotic relationship where they rely on each other for procreation.

Both species are actually hermaphrodites. Genetic exchange between members of the same species happens separately from procreation. For the actual "making babies" process (however you want it to happen), both species are required and the produced offsprings are of both species (or alternatively there could be different procreation processes for creating males and females, and they perform whichever the society currently needs more of).

Culturally they would appear to be of the same species. They might even think of themselves as the same species. It would take advanced biology knowledge to reveal the truth that they actually aren't.

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Functionally you can have any different appearance you want, so long as the males and females have a compatible set of sexual organs (won't even need those if the males fertilize eggs after they have been excreted by the female) or at the very least a set of attractants to they can find each other and realize they can mate with each other (and you STILL don't need even that if the male releases his sperm into the environment and the female just stumbles across it!).

While the male and female may differ only by a single sex chromosome in the tried and true XY type sex determination system, that chromosome can enact all sorts of changes by up or down regulating all the other genes. And if you get into more complex sex determining systems, like the temperature system of some reptiles, you could affect all sorts of genes. Those trilobite beetles, for example, may represent a female that is stuck in a larval stage while the male molts and turns into the winged beetle form.

So you are really limited only by environment. It is unlikely, for example, that an aquatic female would ever encounter a terrestrial based male, unless they both frequent tide pools or something. But there really is nothing that keeps a hummingbird-like male from mating with a grizzly bear-like female assuming they share the same territorial range and have some mechanism for exchanging DNA. The insect world has many examples of extreme sexual dimorphism.

But the key here is that the reasons for sexual dimorphism have to make sense. The more care and raising of offspring needed, the more similar to each other mates tend to be. If the young are all on their own from the get go the appearance of the parents is functionally superfluous and the males and females can resemble two totally different species but if the parents care for the offspring then the sexes tend to be pretty close in appearance (at least when young) so they can have similar "child care" requirements.

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This is a pretty unanswerable question, given the complexities of genetics. But from a basic standpoint: As much as you like, as long as the two different genetics are still capable of being combined to form another creature capable of reproducing.

In you scenario you specifically asked about male and female, so I'm assuming binary sexual reproduction here (two partners, two sexes). These two will (if they're anything like us. Who knows, they might not be!) have a matching number of chromosomes. Without a matching number they might still be able to have children, but those children would then not be able to procreate further.

Aside from that you can go wild with genetics. You can't guarantee that the children of this coupling would be in any way the same as their parents, or even that they'd survive particularly long, but fundamentally your question boils down to whether or not the chemicals in one parent's gametes can successfully combine with the chemicals in the other parent's gamete and bring together some other chemicals that like to keep replicating.

In the case of humans that's hideously complex, but essentially boils down to half of a stable chemical arrangement from one cell being fastened onto half of another stable chemical arrangement from another cell. If the combination of those is another stable combination of chemicals another stable chemical arrangement that replicates itself occasionally then you've got reproduction.

So go nuts. It's your world. And unless your world is going to be a rather dull place full of discussions on diploid pairing and zipping proteins you can pretty much forget about actual genetics and just focus on the macroscopic differences, for which you don't even need differing genetic codes, as evidenced by bees, ants, beetles, spiders, lions and Man-O-War jellyfish.

Have fun!

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