This section is an addition to the Elf Horns Meta Post. With the addition of the following sections, the revised contents for the entire horn post are as follows, with sections covered in this reblog in bold:

1. Introduction

2. a) Anatomy of horns

b) Elves with branched horns

c) Sensitivity of horns

3. Horn development

4. Genetics of horns

a) Glossary of terms

b) Hornless elves

c) Horn placement

d) Elf type crossbreeding

i. Colour inheritance

ii. Texture inheritance

iii. Placement inheritance

iv. Shape inheritance

e) Elf-human crossbreeding

i. Hybrid genetics

ii. Inheritance of horns

5. Possible functions of elf horns

6. Scenarios for content creators

I’ll also be posting the references for this new section at the end.

This section is actually longer than the entirety of the original post, so. Be advised. Total word count for this reblog text alone is about 6500.

Content warnings for this section specifically: Considerable discussion of potential genetic deformities relating to horn and sinus inheritance, as well as possibilities of deficits from hypothetical linked genes. In 4e, there is also brief mention of the hypothetical genetics of intersex hybrids.

4) Genetics of horns

This is kind of a very dense section. My apologies. Genetics sometimes just be like that.

The placement, shape, size, growth, and function of elf horns will all depend on the genetics behind them. As a result, there’s a lot to be said about the possible genetics of elf horns.

4a) Glossary of terms

Most of these definitions will either be explained or can be inferred from the text. Some probably can’t. Refer to this glossary if you get perplexed.

A glossary of terms used in this section:

Gene – a part of an organism’s genetic code that codes for a particular physical trait. For example, one gene may decide what shape an animal’s horns grow in.

Allele – a ‘different form’ of a gene. A gene itself may code a specific physical trait, but genes can exist in different forms. For example, the fur of an animal can often come in different colours and patterns, for which differing alleles are responsible. The different forms a gene can take are referred to as alleles. Alleles are inherited – one allele for every gene in an organism is inherited from its parents. Alleles are usually written as upper or lower case letters, such as P or p.

Homozygous – refers to an organism who inherited two copies of the same allele of a gene from each of its parents. Examples of homozygous allele configuration: PP, pp.

Heterozygous – refers to an organism who inherited a different copy of an allele of a gene from each of its parents. Example of heterozygous allele configuration: Pp.

Phenotype – the observable physical appearance of something, generally of a specific body part or trait. For example, a black-furred dog can be said to have a black-furred phenotype.

Genotype – the not necessarily observable state of an organism’s genome. Sometimes you can determine genotype of an organism by breeding it with one or more other organisms and making inferences from the phenotype of the offspring, but otherwise genotype may not be explicitly obvious to the naked eye. The allele examples given for homozygous and heterozygous are also examples of genotypes.

Linked genes – genes that are likely to be inherited together because they’re located close to each other on the same chromosome. Genes are commonly linked when they are essential to related aspects of each other’s function. It can also happen in genes that have very little to do with each other, like the gene for red hair and the gene for freckles.

Homeobox genes – explained in text, but basically these are the genes responsible for making sure things develop in the right basic shape very early in gestation, with limbs in the right places and so on. You may have heard of HOX genes in some sci-fi, which are the homeobox genes relating to limb configuration.

Dominance (genetic): Refers to how alleles on the same gene interact. Dominance is always relative, and an allele that may be dominant over one type of allele could be recessive to another.

Dominant allele – an allele whose associated phenotype will generally be ‘dominant’ over recessive alleles of the same gene. Dominant alleles are represented by upper-case letters, such as P. It usually only takes inheriting one copy of a dominant allele to see it manifesting in the phenotype of the organism.

Recessive allele – an allele whose associated phenotype will generally be overridden by dominant alleles on the same gene. Recessive alleles are generally represented by lowercase letters, such as p. Organisms with one copy of a recessive allele might still have observable phenotypic differences, but frequently, it takes two copies of the recessive in order to manifest completely – in other words, homozygous recessives are more likely to result in phenotypic differences than heterozygous recessives.

Incomplete dominance – when the alleles of a genotype are not entirely dominant or recessive to each other, and the resulting phenotype lies somewhere between the two. For example, if breeding a red snapdragon to a white snapdragon, you end up with pink coloured offspring.

Co-dominance – when, in a heterozygous genotype, you see evidence of both alleles in the phenotype. For example, bi-colour coats in animals, or flowers with more than one colour on the bloom.





4b) Hornless elves

If elves have genes responsible for making their horns grow, then there is absolutely a chance that you could have naturally-occurring hornless elves. The genetic mechanisms for this and the consequences of it could go a number of ways.

On our own dear Planet Earth, many horned animals have hornless variants, which are referred to as ‘polled’. Because hornless livestock are more convenient to farmers, and because breeding hornless animals is more humane than dehorning, there is a great deal of effort dedicated to intentionally breeding polled livestock.

The genetics of polled animals vary across species, and even within species. In cattle, for instance, the polled allele is dominant, so you can often easily obtain polled variants of a cattle breed simply by breeding a polled bull of one breed to horned cows of another. However, this doesn’t work for all cattle breeds. Genetics of hornlessness can sometimes be more complex than the state of the alleles on a single gene. There is quite a lot of variety in size and shape of cattle horns, so it shouldn’t be surprising that different genes are sometimes involved in hornlessness for some cattle versus others.

In other animals, such as sheep, the polled gene is recessive. In some, such as goats and water buffalo, the polled gene is actually linked to other genes in the body, and hornless animals can have defects in things not related to horns at all, such as their reproductive organs.

I present explanations for both dominant and recessive polled horn genes, but it is important to note: these are not mutually exclusive. There could be two different mutations on the horn genes that will inhibit horn development, or different mutations on different horn genes, or multiples of each. It’s completely possible for multiple types of hornlessness to exist in the population at the same time.

If the elven polled gene is dominant:

The polled allele will be written as P, and the horned allele as p. A homozygous hornless elf is represented by PP, a homozygous horned elf as pp, and a heterozygous elf as Pp.

PP will always result in a hornless elf, and they may not even have horn buds.

Pp will most likely result in a hornless elf, but they may retain horn buds that do not develop. These will never fuse to the skull and will never grow substantially, remaining cartilaginous and malleable for life. These polled horn buds are referred to as ‘scurs’.

pp will result in a horned elf.

Whether or not offspring are born with horns depends on the genotype of the parents.

Genetic outcomes:

a) If PP reproduces with pp, the offspring would be: entirely composed of hornless elves who may have scurs (Pp, Pp, Pp, Pp).

b) If PP reproduces with Pp, the offspring would be: 50% hornless without scurs, 50% hornless with possible scurs (PP, Pp, PP, Pp).

c) If Pp reproduces with pp, the offspring would be: 50% hornless with possible scurs, 50% horned (Pp, Pp, pp, pp).

d) If Pp reproduces with Pp, the offspring would be 25% hornless without scurs, 50% hornless with possible scurs, and 25% horned (PP, Pp, Pp, pp).

e) If PP reproduces with PP, all offspring will be PP, hornless without scurs. If pp reproduces with pp, all offspring will be horned.

If the polled allele is dominant, you might expect this trait to gain more traction in the elf population, unless the individuals involved made a conscious decision not to have children.

If the elven polled gene is recessive:

The polled gene will be written as p, and the horned gene as P. A homozygous hornless elf is represented by pp, a homozygous horned elf as PP, and a heterozygous elf as Pp.

PP will always result in a horned elf.

Pp could have variable outcomes. It could result in elves with scurs, elves with smaller stunted horns, or it could have no visible phenotypic effect at all. It’s completely possible that elves with one copy of the polled recessive may be no different on the outside to PP horned elves. I’ll refer to this as ‘undetermined horn phenotype’ in the genetic outcomes.

pp will always result in a hornless elf, likely without scurs.

Genetic outcomes:

a) If PP reproduces with pp, the offspring would be: entirely composed of ‘carrier’ elves with undetermined horn phenotype (Pp, Pp, Pp, Pp).

b) If PP reproduces with Pp, the offspring would be: 50% horned, 50% with undetermined horn phenotype. (PP, Pp, PP, Pp).

c) If Pp reproduces with pp, the offspring would be: 50% with undetermined horn phenotype, 50% hornless (Pp, Pp, pp, pp).

d) If Pp reproduces with Pp, the offspring would be 25% horned, 50% with undetermined horn phenotype, and 25% hornless (PP, Pp, Pp, pp).

e) If PP reproduces with PP, all offspring will be PP, horned. If pp reproduces with pp, all offspring will be hornless.

Depending on how carriers of the polled gene manifest, it’s very possible that there could be a lot of elves walking around with no idea that they’re carriers. If they had children with another carrier, they might well end up with a hornless child from the union. You might, alternatively, expect carriers to be aware of the fact that their family line has had hornless individuals in it, and to possibly be wary of having children with an elf with a similar family background.

On the whole, if the elven polled gene is recessive, you can expect to see fewer hornless individuals in the population.

Linked genes:

It’s certain that elves with horn sinuses will have the genes responsible for the sinuses linked to the genes responsible for horns. This will be discussed in more detail in the next section.

It’s possible that, like in some earth animals, the horn genes are also linked to seemingly unrelated aspects of elf physiology. This could be pretty much anything, from skin pigment to reproductive organs to skeletal development. Depending on what genes the horn genes might be linked to, hornless elves could potentially encounter deficits in basically anything. Hornless elves could even have fatal deformities that prevent them from surviving. If canon does not elect to give us any hornless elves, you could use this as one very depressing explanation for why that might be the case.

Summary of Hornless Elves:

Since elves have horns, hornless elves are very likely to exist as a possible mutation on the horn development gene/s. How likely this mutation would be to spread would depend on how dominant the hornless gene is, and also how likely hornless elves are to reproduce.

If the horn genes are linked to other genes, hornless elves could have any number of potential problems. The list of potential problems is as long as the list of functions of the body, so I’m absolutely not going to write it.









4c) Horn placement:

Different types of elves have horns placed at different points on their skulls, and therefore fused to different bones of the skull.

Frontally-fused types: Skywing, Sunfire.

Parietally-fused types: Moonshadow, Startouch. Note: all figures in the original post feature parietally-fused horns.



Temporally-fused types: Hypothetical elf whose skull we see in s1e2*

*In s1e2, our first scene in Viren’s dark magic workshop shows us what looks like an elf skull on one shelf. The horns of this skull protrude from around where the ears would be, on the temporal bone, and they arch forwards. This skull is not there in later shots of the same scene in s1e3, so it’s unclear whether this should be considered canon or not. It could also potentially be the skull of some sort of horned humanoid animal, such as a horned monkey or ape.

Note, Sunfire horn placement seems almost on the verge between parietal and frontal. I might hesitantly call it prefrontal placement.

Now, the placement of elf horns is most likely determined by what we call homeobox genes. These are a family of similar genes that direct the formation of several basic bodily structures during very early gestation. A homeobox is a sequence of DNA found within genes involved in the patterns of anatomical development in animals (also fungi and plants, but less relevant here). Fun fact: differences in homeobox genes are responsible for the difference in finger and toe counts between humans and elves, as well as the presence of horns versus no horns.

The MSX homeobox genes are the ones responsible for determining the development and structure of the head, face, and teeth. As a result, I would expect elves to have differences in their MSX genes as compared to humans. I would also expect to see differences in these genes between types of elves with differing horn placements. I’m no geneticist, so I’m not going to give examples of additional homeodomains coded for in elves that might be responsible for horns. Instead, I’ll make some reasonably educated assumptions:

- The MSX homeobox genes tell the foetal elf’s cells where the horn buds should be growing.

- If elves have horn sinuses, the genes for these will be linked to the genes for horn placement, and their development will also be directed by the MSX homeobox genes.

The most interesting thing to consider about the differing position of elf horns is how their sinus structure might also differ – assuming, of course, that elves have sinus cavities in their horns. I see two main options here:

1. Elves all have the same basic sinuses: frontal, maxillary, ethmoidal, sphenoidal, prefrontal (see section 2), and parietal (see section 2). Depending on horn placement, some of these will develop more dramatically than others. For example, a Moonshadow elf’s parietal sinus will be larger and more developed to extend into their horns, whereas this sinus might be as small as the ethmoidal sinus in a Sunfire elf. If we assume the existence of elves with temporal-fused horns: the number of paranasal sinuses might be one larger, with the addition of the temporal sinus. This would be located around the ear, likely slightly above or behind it, but further into the skull. It would drain to the sphenoidal sinus and from there into the nasal cavity.

2. Elves will develop or not develop certain sinuses depending on the horn placement gene the sinus development genes are linked to. For example, a Skywing elf may not develop the parietal sinus at all. This early differentiation would likely take place in the womb, possibly during very early embryonic development.

I find both equally compelling, with perhaps a slight preference for option 1. It all depends on what genes are responsible for the positioning and development of both horns and sinuses.

With option 1 in mind, I posit the following differences in sinus development between elves:

Moonshadow and Startouch elves: parietal sinus connects to the horns, and is significantly larger than in other elves. Prefrontal sinus may also be larger, to enable better drainage of the parietal sinus into the nasal cavity.

Skywing and Sunfire elves: prefrontal sinus connects to the horns, and is significantly larger than in other elves. Alternatively, it could be the frontal sinus that connects to the horns, but I consider this less likely, as the frontal sinus is much lower on the forehead than the horns, and I would sort of expect these elves to have bulging foreheads if their frontal sinuses were that large.

Hypothetical temporal-horned elves: Given the closeness to the sphenoid bone, I feel that the sphenoidal sinus could connect to these horns. Alternatively, as stated before, these elves might have an additional sinus, the temporal sinus.





With option 2 in mind:

Moonshadow and Startouch elves: these elves would be the only ones to develop a parietal sinus in utero.

Hypothetical temporal-horned elves: the hypothetical temporal sinus would develop in these elves only.

This might lead to some interesting differences between elf types, as well as interesting implications for hornless elves. Please bear possibilities 1 and 2 in mind as you read the following subsections.

Implications for hornless elves:

This section mainly applies if you consider elves to have horn sinuses.

In the case of options 1, where all elves develop the same sinuses but to varying extents, your hornless elf could end up with a number of different sinus configurations. They’ll have all six/seven paranasal sinuses, but the allele that deactivates horn growth may or may not deactivate the associated sinus growth gene.

As an example: imagine a Moonshadow elf born without horns. The allele that stopped their horns from developing may not affect the linked sinus gene. As a result, their parietal sinus may grow and develop as if the horn was there, leading to cranial deformity later in childhood, as with a dehorned or disbudded elf child. I would consider this most likely to occur if the hornless elf was heterozygous, with only one copy of the polled allele.

If the hornless elf is homozygous, with two copies of the polled gene, I consider it more likely that the parietal sinus either wouldn’t develop beyond its small base state (Option 1 genetics) or wouldn’t develop at all (Option 2 genetics).

Overall, I would assume that polled elves would not experience cranial deformity, simply because polled cattle don’t. Unlike dehorned and disbudded cattle, polled cattle do not present with engorged craniums from sinuses that try to develop but have no room. Unfortunately, there isn’t a lot of literature suggesting why this is – it could be that the calf’s horn-connected sinus is present but doesn’t develop further (most likely), or that the sinus is entirely absent (less likely).

With option 1 in mind, I would expect a homozygous polled elf to have all the usual sinuses, but at sizes indicative of the lack of connected horns. In a heterozygous polled elf, it’s possible that their recessive might be linked to a gene that allows the associated sinus to grow, leading to cranial deformity.

Summary of Horn placement

Different types of elves have horns growing from different points on their heads. The genes most likely responsible for determining this are called homeobox genes. These horn placement genes would be linked to the sinus genes, meaning that it would be fully possible for elves with horns in different locations to still develop sinus cavities in their horns.

Genetically-hornless elves could potentially encounter problems if their sinus genes are not also suppressed by their hornless genes.





4.d) Elf type crossbreeding

If different types of elf interbreed, this could have a large number of outcomes for their horns.

Different types of elves have very different horns. They seem to vary in colour, texture, shape, and positioning depending on type, which means that inheritance of these traits could differ in hybrids.

i. Colour:

Horn pigment would probably be influenced by the keratinocytes (keratin-producing cells) found in the dermis and epidermis of the horn. Most horns in Earth animals tend to be black, grey, brown, or reddish-brown. There is really not a lot of information to be found about the genes determining pigment in animal horns. However, there’s a great deal about the pigment genetics of a different type of keratin: human hair. In human hair, pigment is largely determined by the amount of different types of melanin in it. I feel that elf horns could easily work the same way.

The horn colour inheritance would depend on how the horn colour alleles interact. Some horn colours may be dominant or recessive to others, even within the same elf type. For example, brown horns may be dominant to red horns in Sunfire elves. It’s impossible to predict how allele dominance might work out in cross-type hybrids, so you could make this go either way.

If the genes responsible for horn colour are incompletely dominant, then it’s possible that your hybrids may present with colours that are somewhere between those of their parents. For example, a Sunfire-Skywing hybrid with dusty red horns.

If the genes responsible for horn colour are co-dominant, you could even end up with horns that have patchy or variable colour – bicolour horns are a real possibility. This would be more likely to take the form of random patches of one colour than any sort of pretty gradient, though.

Summary: Depending on how the colour genes work and interact, elf crossbreeds could inherit a colour from one parent, a mixture of the colour of both parents, or even a patchwork combination of the two.

ii. Texture:

Horn texture would be influenced by the villi structure of the dermis and epidermis within the horn, as these produce the keratin, and the structure of their villi determines the appearance of the keratin. Moonshadow elves seem to produce keratin with length-ways ridges in loose curving shapes. Sunfire and Skywing elves seem to have smoother horns. This will be due to differing structure of the villi in the horn skin.

Much as with colour, we can consider dominant/recessive, partial dominance, and co-dominance to decide how texture is inherited.

Keratin texture may be determined by alleles on the same gene that are dominant or recessive to each other. In this case, hybrid offspring will inherit one parent’s texture or the other’s.

If the alleles are incompletely/partially dominant, you might end up with a texture somewhere between the parents’. Please note that this might not be structurally stable for the horn: horns with mixed textures may break more easily, chip or scratch more easily, or have more points of weakness.

If the alleles are co-dominant, you could end up with some crazy mix of the two. Please note that this might not be structurally stable for the horn: horns with mixed textures may break more easily, chip or scratch more easily, or have more points of weakness, especially in areas where one texture lies alongside another.

Depending on how the genes are represented, and if there are multiple genes involved, crossbreeds could be born with entirely new horn textures that may or may not be structurally stable.

Summary: depending on how the texture genes work and interact, elf crossbreeds could inherit a keratin texture from one parent, a mixture of the textures of both parents, or even a patchwork combination of the two. The last two options could be problematic for the offspring – it’s possible that some combinations wouldn’t be well-designed, and could lead to the horn keratin breaking or being damaged more easily.

iii. Shape and size:

Different types of elves have different horn shapes and sizes. Some, like Startouch elves, have entirely different structural configuration, what with their prongs. This would be determined by the MSX homeobox genes. As such, inheritance of horn shape is a bit trickier to work out, as it depends heavily on how horn shape is represented in the homeobox genes, and whether these genes are consistent among elves, and also how many genes are involved with it.

For this I can really only throw possibilities your way and let you choose, because I’ve no clue which is most likely.

- Hybrid offspring could inherit the shape/size of one parent or the other with no changes or alterations.

- Hybrid offspring could inherit a shape/size somewhere in between – for example, a Sunfire-Moonshadow hybrid with horns pointing back-and-up, curved or straight. This would have a chance of deformity, with poor combinations of traits leading to horns that don’t grow correctly, are stunted, or anything along those lines.





- Hybrid offspring could present with a shape/size wholly different to either parent due to strange interaction of genes. This could be cool and interesting, or it could be a huge problem. This is where deformities could potentially occur. Weird interaction of size-related genes, for example, could lead to offspring with horns that are far, far too large, probably leading to significant neck problems and possibly over-developed sinuses. Problems with horn shape inheritance could also lead to deformities within the structure of the horn itself – the bone might not develop properly, or be properly vascularised, and so on.

Hybrids of Startouch elves and others might encounter a unique factor here: whether or not the offspring has branched horns, they’d have a chance of shedding their keratin periodically.

Summary: Outcomes for the shape and size of hybrid horns are very, very heavily dependent on information I don’t have about how elf horn genes are represented. So the possibilities are: hybrids could inherit shape and/or size from one parent only. They could inherit a combination of the shape and/or size of their parents’ horns, which may not be well-formed and healthy. Or they could end up with dramatically differently shaped and sized horns, which could be cool and awesome-looking, but would have a much higher chance of horrible deformity.

iv. Placement

As discussed at length in 4c), horn placement depends on the MSX homeobox genes. This is the most likely point at which you’ll see deformities or problems with hybrid offspring, as it relies on linked genes being inherited properly.

I think the main possibilities here are as follows:

1. Horn position is inherited wholesale from one parent or the other, along with the linked genes for sinus development.

2. Horn position of hybrids can be somewhere between that of the parents.

2 is where we see potentially very large problems for the offspring.

While the sinuses and their development might be very neat and tidy for the ‘pure’ subtypes of elf, there’s no guarantee that this would hold true if they produced offspring with abnormal horn placement. If a Moonshadow-Skywing hybrid is born with horns somewhere between the parietal and frontal bones, which sinus will connect to it? Will the parietal sinus attempt, unsuccessfully, to grow into a horn that isn’t in the right place? Will the prefrontal sinus attempt to grow into the horn, and successfully pneumatise it? Will both prefrontal and parietal sinuses grow to full horn-penetrating size? Will the sinuses be placed differently, to allow optimal growth into the horn?

Again, the concept of horn sinuses provides a potential point of contention for hybrids. If the sinuses don’t form in a way capable of connecting to a poorly-positioned horn, you could end up with bulging craniums, or hollow horns disconnected from the respiratory system, or any derivation therein.

All told, I feel it would be much better for the health of inter-type hybrids if 1 is the possibility that holds true here. As such, I’m going to call this the likelier option, because I don’t think it would make sense for different elf types to have difficulty interbreeding.

Summary: if you love your hypothetical elf subtype hybrids, please assume that they will inherit the horn placement of either one parent or the other. If the horns end up being located somewhere between, I can only imagine what problems that could cause for them. And I did imagine. Extensively. I even wrote it down.









4e) Elf-human hybrids:

i. Hybrid genetics

Note: this section assumes that humans are native to TDP world, and were not transplanted there thousands of years ago by weird dimensional shenanigans. It also assumes that humans and elves are closely genetically related enough to interbreed, an assumption which has its own implications.

Now, this post is about horns, so I really didn’t intend to go into hybrid genetics more than I had to. But that ship has well and truly sailed, so…here I go talking about hybrid genetics, specifically as they apply to hypothetical elf-human hybrids.

To begin with, if elves and humans are capable of cross-breeding, this means they must have a fairly ‘recent’ common ancestor, and elves and humans would probably be part of the same taxonomic genus. There are known examples of hybrids occurring between different genera, and very rare examples of hybrids between different families, but you can absolutely guarantee that anything further removed than interspecific (hybrids between two species of the same genus) would be infertile.

In this case, the early ancestors of both elves and humans would be different from the early ancestors of real-world humans. This means that humans may be physiologically different in TDP than in the real world. Honestly though, I would just hypothesise that members of the homo genus and their ancestors in this universe had horns, up until whenever humans or humans’ ancestors split off from the horned members of the genus. This seems likely enough, given there are examples of what seem to be horned possibly-mammals in TDP’s opening. Which leads me to the point:

Given that real-world humans did interbreed with other members of the homo genus around 47k years ago, I consider it very possible that all humans have a low percentage of elf DNA, and elves a low percentage of human DNA. In our world, we have about 1-4% of non-homo sapiens DNA, but the other members of the homo genus died out a long time ago. In TDP, therefore, there could be a much higher proportion of mixed DNA around, though of course that will have lessened in the last thousand years. Given their relative lifespans, elves in current events probably have more human in them than humans have elf. Ironic, really.

If modern elves and modern humans have the same number of chromosomes, and are part of the same genus, they might be able to have fertile offspring. If they have different numbers of chromosomes, there’s basically no chance their offspring will be fertile. If there is any chance at all, it’ll be for female hybrids only, who may be able to breed with a male human or elf (but possibly only one of the two, rather than either species). As a general rule: female hybrids will be much, much more likely to be fertile than male hybrids, due to the sensitivity of the sex-determining genes on the Y chromosome. Hybrids with XY chromosomes may be more likely to be intersex or have hermaphroditic traits than females, and will almost never be fertile. For more information on this, refer to ‘Haldane’s Rule’, linked in the references.

Historically speaking, if there ever was crossbreeding between ancient elves and humans, this would be how elf genes entered the human genepool and vice versa: female hybrid offspring would breed back to a male of one of her parent species, produce fertile offspring, who would then breed with more members of the species of their father, and so on.

You would, on the whole, have two categories of hybrids: male elf + female human (elman?), and male human + female elf (helf? Humelf? I’ll go with ‘helf’, as it’s funnier).

An elman would have the mitochondrial DNA of a human, and a helf would have the mitochondrial DNA of an elf, as mitochondrial DNA is 100% matrilineal.

Summary of Hybrid genetics:

In TDP, humans and elves are almost certainly members of the same genus, which means they’ll be closely genetically similar. Quite possibly close enough to interbreed and produce fertile offspring. In the case of elf-human hybrids, your hybrid females will probably be the fertile ones. Male first-generation hybrids may be infertile, or intersex, but will be unlikely to be able to breed.

Now, finally, onto actual horn inheritance:

ii. Horn inheritance

Given that humans and elves share common ancestors, humans could be considered hornless elves more than elves could be considered horned humans. As such, humans could be considered polled for the purposes of thinking about hybrid genetics; in other words, some ancient humans would have been born hornless, most likely with this as a dominant gene, and horns would eventually have been bred out of the population. Depending on how long ago this was, it could be possible for humans to very, very rarely be born with naturally-occurring horns, which I imagine wouldn’t go terribly well for them or their parents in the modern political climate.

This does make things simpler for elf-human hybrids, however. Given the shared genetic heritage, there might well be room in the human MSX homeobox genes for horn-related genes. If the genes are similar enough, it could just be a matter of the polled allele encountering the horned allele: the human could be considered PP, the elf pp, and their offspring would be Pp, Pp, Pp, and Pp. In this case, you might expect all hybrid offspring to be hornless with scurs. Hybrid female offspring, Pp, would then be able to breed back to a male elf and produce offspring with horns, pp.

I consider it extremely unlikely that the human polled gene would be recessive, as otherwise humans with horns would be relatively common, or at least not unheard of. The only way horns wouldn’t be prevalent in the genepool is if something happened in antiquity to wipe out all or most individuals with the dominant horn allele.

Now, because you and I both probably enjoy the idea of horned hybrids, I’ll put forwards some other possibilities.

1 The human polled gene has differentiated enough from the elf horn gene that when the alleles meet, you encounter principles of incomplete dominance. This would mean that when you end up with a Pp heterozygous hybrid, neither the P or p allele would be fully dominant, and the phenotype would manifest somewhere between the two. So in this case, your elf hybrid would probably have small, stunted horns, or at the very least shorter horns than their elf parent.





2 If the human polled gene and the elf horned gene are co-dominant, this means that signs of both alleles would manifest in the hybrid offspring, which could end up being extremely messy. This is probably the possibility most likely to result in deformity, but it could also have exciting possibilities such as hybrids with a single horn. But, on the other hand, it could result in horrifying deformity. Examples: some parts of the horn genes functioning and others decidedly not functioning. There’s a lot of ways this could go and many of them are highly unpleasant, especially when we think about linked sinus genes.

All things considered, you don’t want your horn alleles to be co-dominant in hybrids unless you’re interested in creating a story where this is one of the reasons elf/human relationships might be forbidden: the children often have a very bad time, genetically speaking.

If you want cute healthy hybrid kids, please assume either dominant polled allele for hornless kids, or incomplete dominance for horned kids.

Everything horn-related that the hybrid does inherit will be from their elf parent: shape, texture, colour, and so on. Unless – and this is a really small possibility – enough of the human colour/shape/texture-determining genes linger from antiquity in the genome, in which case the offspring could have cute stunted horns in colours and shapes and textures not seen for tens of thousands of years.

On sinuses:

Depending on genetic heritage, humans in this setting might well have extra paranasal sinuses themselves. If they don’t, then inheritance of horn-related sinuses would likely be linked with inheritance of horns in general, as these genes are strongly linked. I consider it quite likely that TDP humans would have the extra sinuses, even if only as small vestigial cavities.

On the whole, though, I consider sinuses one of the greatest potential points of deformity in these hybrids. If you have a Moonshadow-human hybrid, for example, everything could align nicely and the incomplete dominance could give you a child with small horns, functional parietal sinuses, and functional miscellaneous paranasal sinuses. But…on the other hand…

On the other hand, it could go wrong. It could go wrong in many ways. The parietal sinuses might develop properly, but the prefrontal might not, leading to partial or complete isolation of the parietal sinus from the respiratory system. The drainage channels between all the sinuses could be malformed, leading to a child who is very likely to experience frequent sinus infections. The homeobox genes for the sinuses could get mixed up, and cause all potential horn-sinuses to develop to full size, leading to very dramatic cranial deformity as the child ages.

If I wanted to create a hybrid child character who was mostly healthy, had horns, but did suffer some health problems, I would probably choose the drainage option. Then you’d have a kid who got sick quite a lot, which would be great for angst, but wouldn’t have as much risk of outright mortality or horrifying miscellaneous problems.

Summary of Horn inheritance

For the purposes of talking about hybrids, humans can be considered to have the ‘hornless’ gene rather than no horn gene at all. This means that a human breeding with an elf could work in the same way as a hornless elf breeding with a horned elf – at least, as far as the horns are concerned. Hybrids would either be born hornless with non-developing horn buds, or with small stunted horns that would likely be the same colour, texture, shape, and placement as their elf parent’s.

Sinus layout would also likely be inherited from the elf parent. If it wasn’t, or if not all of the linked genes came through properly, this could lead to serious problems for the hybrid.

Afterword: I spent all day doing this. Sure, I enjoyed it, but, well. I hope you’re happy, my dear readers? I’ve only read over this once so there might be typos in places, this thing was a mess a while ago and remnants of that mess might yet remain.

I think it’s very likely that I’ll be formatting the whole post to put on ao3 eventually., as ao3 is much easier to retroactively edit to add new sections or clarifications and so on. For the purposes of tumblr, any future additions will just be reblogged in a giant post train.

References for this horn genetics section:

My partner: a biochemist who I threw a lot of concepts at, and from whom I received useful terms such as homeobox genes and co-dominance in exchange.



Wikipedia: polled livestock

Article about polled genetics

Another article about polled genetics

Yet another article about polled genetics

An article about allele inheritance

Genetics of hair colour

About homeobox genes

About linked genes

Wikipedia: linked genes

MSX homeobox genes

Wikipedia: human evolution

About hybrid biology

About what happens when hybrids make a new species

Haldane’s Rule

Wikipedia: Mendelian inheritance

Wikipedia: interbreeding between archaic and modern humans

Wikipedia: reproductive isolation

Wikipedia: dominance (genetics)