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Think of a species (ONLY ONE SPECIES) the individuals all live in the same environment, share the same sentience level BUT are divided into "A", "B" and "C" races which have different number of limbs, eyes, natural weapons, etc.

For example A has tentacles, B has bird legs and feet, C has horse legs, this sort of different.

They are able to interbreed and have fertile hybrid offspring, but the hybridization would last only for one generation, like if A and B have an offspring, it will be an hybrid AB, now if AB has offspring with A, all their children will be pure A, if AB has offspring with B, their children will pure B.

If an AB breeds with C, their offspring will be AC or BC hybrids. (AB +AB = 25% chance of pure A 25% chance of pure B, 50% chance AB hybrid).

Are aliens that have this sort of genetics, crazy different appearances, and only 1 generation lasting hybrids, possible?

Can this be explained by epi-genetics?

Is this possible at all?

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  • $\begingroup$ Have you done any research? What possibilities have you considered and discarded? Hint: very occasionally, female mules give birth to offspring. What is the foal of a female mule? A donkey, a mule, or a horse? Can you imagine a mechanism to mitigate the rarity of fertility in mules? $\endgroup$ – AlexP Aug 31 '19 at 12:10
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The early hominid period on earth had many highly intelligent species of ape existing in a relatively narrow band of time and often overlapping with one another, so that part of this question isn't so far fetched. If things had gone different and modern humans had proven to be less successful than we were, it isn't so hard to believe that there would have been a period where humans co-existed on this planet with Neanderthals and the like. Hybridization between modern humans and Neanderthals has also long been theorized as having been possible, so that portion of your theory is also somewhat possible.

However, it gets more complicated when you talk about things like not inheriting the traits of one's grandparents. That's very unlikely, and I'm not even sure it's possible at all. Even we contain the genetic remnants of ancient retroviruses that ravaged animal populations in the distant past, so the idea that somehow the grandparent's genes just aren't inherited at all is difficult to accept.

Bringing epigenetics into is an interesting idea, but I'm not sure it would solve your problem, at least not to the vast extent you're looking for. The changes in gene expression from epigenetics, at least as my very limited knowledge on the subject indicates, tend to be much more subtle than, say, causing a chitinous exoskeleton to grow in place of normal body hair.

The idea of a number of closely related species with vastly different traits, however, is not as out of the question. For instance: coyotes, grey wolves, and domestic dogs can all reproduce with one another and produce viable fertile offspring, despite the vast difference between say a wolf and a basset hound. However, some of the differences you describe would almost require organisms that fit into completely different species, assuming these alien organisms play by the same rulebook as earth. Boneless tentacles are genetically far different from horse legs. Gills and bug wings are almost more closely related (bug wings having ostensibly evolved from modified gills).

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  • $\begingroup$ Well, one can imagine that all individuals have the genetics to exhibit all traits, and the specific package is selected by one master switch locus. $\endgroup$ – AlexP Aug 31 '19 at 12:13
  • $\begingroup$ That's a possibility I didn't think of. I still think the likelihood would be low.. it's just a lot of genetic information to pack into one being with a high chance of going wrong, but there exists a possibility that I can't really comment on too much. $\endgroup$ – Benjamin Wandio Aug 31 '19 at 12:28
  • $\begingroup$ Perhaps I'm reading you wrong but modern humans and Neanderthals did coexist and they did interbreed. Anyone with ancestry outside of the southern 2/3 of Africa has Neanderthal DNA, usually in the 2-4% range. Ditto with some of the other hominids. en.wikipedia.org/wiki/Neanderthal_genetics $\endgroup$ – Cyn says make Monica whole Sep 2 '19 at 15:16
  • $\begingroup$ Re-read my first paragraph: That was me supporting the theory that multiple intelligent species could exist together. ;) I posited that hybridization IS possible, it's the losing of traits after a single generation that seem less plausible. $\endgroup$ – Benjamin Wandio Sep 3 '19 at 10:30
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This arrangement is very possible and we don't need anything like epigenetics to explain it either. Your idea can be explained by classical Mendelian genetics, all we have to do is say that your alien species has two copies of a single, large chromosome.

To explain how this will work let's first go over some basics. Most genomes of multicellular life on Earth are fragmented into many chromosomes which are all inherited randomly. You have two copies of any given chromosome, one from your father and one from your mother. Your parents each had two copies themselves and random chance dictated which of those two copies you happened to inherit. Since we have 23 unique chromosomes that means each new generation represents a shuffling of alleles and traits. Because there are so many chromosomes and because of other mechanisms like crossing over that further shuffle alleles each subsequent generation is a mixture of its parents' genomes. So you have roughly one-half of the genes of your parents, and roughly one-quarter of the genes of your grandparents, etc. With each generation, the many alleles in our genomes continue to mix with nearly infinite possibilities.

But what if instead, we had two copies of only one large chromosome which never crossed over? Now you would have discrete genetic possibilities without the potential for further mixing. So when your hybrids reproduce they won't produce further hybrids, rather their offspring will be either be hybrids (heterozygous) or revert back to purebred (homozygous). This is almost exactly the system you described above with one minor change. An AB individual reproducing with a BB individual would result in half of the offspring being AB and half being BB.

Basically, your organisms function as if they have only a single monoallelic trait. We often examine these systems using simple Punnett squares. By forcing the entire genome to be inherited as a single unit the whole organism now only has one "allele" and so all of the crosses you can make will obey the laws of the simple Punnett square and classical Mendelian inheritance.

You aren't limited to just 2 or 3 of these chromosomes either. You could have dozens of unique haplotypes if you wanted.

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It is in theory possible for something like your arrangement to exist in a species. It wouldn't last long and has horrible consequences for a lot of things, though.

Your species will require its morphogenic (body-planning) genes to be on a single chromosome. This might work, but is obviously unnatural. This chromosome is the "race" chromosome, similar to the sex chromosomes in real animals.

This means that an AB "hybrid" (race chromosomes AB) breeding with a purebred B (race chromosomes BB) has a 50% chance of another AB hybrid, which violates your "no grandchild hybrids" rule. This system also requires genetic infrastructure to stop gene shuffling between different race chromosomes.

However, this idea is dumb. See, your critters are going to have different requirements depending on what niche in the ecology of their environment. Something like a squirrel that lives on nuts and berries will require a different digestive tract compared to a predator, and both have different systems compared to a cow. A burrower is going to require different senses as compared to an airborne predator or an airborne scavenger. You're going to have to move so much on to the race chromosome that you're going to put the entire genome on it.

And hybrids are going to be a dead end. What do you get when you cross a buffalo's body plan with a wolf's? Something too slow to hunt but too small to be a ruminant (and therefore graze.)

Hybrids aren't going to happen either. Herbivores aren't going to want to mate with predators, and predators would eat herbivores rather than breed with them. The most available mates would likely be that of the same race.

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Evolution and natural selection often lead to animals having physical barriers between themselves and other 'evolutionary branches' of themselves: while zebra's and horses can have offspring only the most widely abnormal circumstances would ever lead them to one another in the wild. This is normally enough to guarantee sexual isolation inside a single species. This is prezygotic isolation, stopping the hybrids before they start.

However, this barrier is sometimes not enough for species which have branched off but remain together in the same environment: like yours.

This is where postzygotic isolation comes in: stopping the hybrids thriving, because (and to anthropomorphise evolution a bit) you'd be surprised how much evolution doesn’t want its species mixing and matching. These hybrids are often given either obvious or hidden weaknesses: sterility, and limb, internal, or mental defects.

The way you’ve clear cut their possible genetic matches is most likely too specific, nearly impossible as genetics aren’t that pre-cut and pre-defined between 1 generation. So, your first hybrids will be fertile but will most likely have physical defects and weaknesses that comes with the mixing of such different species. They would also most likely be undesirable mates and have trouble passing on their genetics anyway.

Their offspring however, now only a quarter of another species (to simplyify things) will most likely be better off. They will be look like a ‘pure’ one of their species however on the inside they are filled with differences that are not obvious on the outside. Depending on how advanced your species’ are (can they test genetics?) they will look like any other pure one of their species and without the ability to further test the individuals, that’s probably good enough for your guys. Giving only the appearance of 'one generation hybrids.'

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I can think of two mechanisms to keep your 'Crazy Genetics' model self-sustaining.

They both deal with the species RNA.

The first mechanism is to assert that the RNA encoding only supports replication of A, B, C and pure combinations of those genotypes. When RNA encounters a hybridize form that would result in a non-pure (1/2 A and 1/2 C genes, for example) the RNA function stops, jams in a way, or stops operation entirely. The consequence of this is that mitosis and cell division would be impossible, meaning that any offspring of an AB parent and C parent will only be viable is its genotype is AC and other possible combinations would be non-viable in utero.

The other mechanism is similar, but the RNA replications the missing fraction of the genetic code using an internal encoding inherited intact from the mother. This mechanism would only be invoked in the case of the offspring was potentially hybridized, then it would make offspring have the same genotype as the mother.

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  • $\begingroup$ A variant on the second mechanism would be to have mitochondria A, B, and C as well, and some interaction such that each mitochondria variant only functions correctly when the cell has a copy of the corresponding nuclear variant. In a hybrid, the half of the germ cells with the father's nuclear variant would be inviable. $\endgroup$ – Peter Taylor Sep 2 '19 at 16:35
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Something similar is happening in nature through not as crazy as you implied, just look at ants or bees, their queens have more or less DNA with their workers, but look totally different.

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