10
$\begingroup$

In a non-magical world where homo sapiens has evolved into several races or breeds (like dogs) or even species as in Q10102 and Q12190, that can interbreed for the most part, how can one model the chromosomes or genes to restrict the possible outcomes so each offspring clearly belongs to one (possibly hybrid) race?

Requirements

  • Female and male offspring of the same parents may have different races,
    e.g. human mother + vampire father = vampire daughters, human sons.
  • A female of one race can be impregnated by a male of another race, but it fails with switched races or sexes,
    e.g. vampire mother + human father = no offspring.
  • The race of certain offspring may be determined by the race of the parent with the same sex,
    e.g. dwarf mother + gnome father = dwarf daughters | gnome sons.
  • Some races are restricted to a single gender and rely on other races for reproduction,
    e.g. sphinx mother + centaur/minotaur father = sphinx daughters.
  • Some hybrid races are infertile when mating with each other, but can reproduce with members of other races (or at least one sex can),
    e.g. human + elf = half-elves, human + dwarf = half-dwarves, both are considered halflings, halfling + halfling = no offspring, but halfling + human = humans (or maybe halflings again).

Which results can be modeled with genetic variety on certain chromosomes and which are better done with different chromosomes?

Q143 “Explaining half-breeds in a world with multiple races” only deals with parts of this question. I guess I’m looking for a general set of rules or algorithm to use.

My current crossbreed design is tabulated at Conworld Wikia. It’s not yet specified whether it’s the result of natural evolution or of some kind of genetic engineering.

$\endgroup$
  • 1
    $\begingroup$ Wow... Simply, wow. Welcome to the site! This is some very fine detail, more than I've ever really wanted to include in my worldbuilding. +1 for the great question! $\endgroup$ – Frostfyre Aug 26 '15 at 13:17
  • $\begingroup$ Hi btw all hinnies and mules are sterile as their total chromosomes didn't match horse or donkey. $\endgroup$ – user6760 Aug 26 '15 at 13:57
  • $\begingroup$ It sounds difficult to make centaurs, vampires and gnomes exist and reproduce with humans in a non-magical world. Are you sure you don't want to include any magic in your universe? $\endgroup$ – Babika Babaka Aug 26 '15 at 14:31
  • 1
    $\begingroup$ Much more depth than I am able to tackle at the moment, but wanted to add a factoid. The y chromosome, which takes the place of an x when present, does not have a buddy for repairs. It actually is a palindrome that can fold itself in half and repair from the data in the other end. $\endgroup$ – DeveloperWeeks Aug 27 '15 at 23:05
  • 1
    $\begingroup$ Looking at your chart, you seem to be building your world with a mostly top-down approach - starting with the setting you are looking for and trying to find an explanation from there. Given your bend toward scientific explanations, it might be better to use a bottom-up approach. Determine the internal physical and genetic traits that make each race different, and derive the implications of the world from there. $\endgroup$ – IndigoFenix Jul 11 '17 at 10:52
6
$\begingroup$

At first glance many of your requirements seem doable, but upon further thought it’s really very difficult to come up with reasonable explanations for them. That said, nothing is impossible, it’s just the explanations might not exist within the realm of classical genetics. I’ll go through each of your requirements and explain why simply having extra chromosomes or dominant alleles won’t be enough to explain the phenomenon. I’ll also give some theoretical considerations to non-classical genetic explanations.

Female and male offspring of the same parents may have different races, e.g. human mother + vampire father = vampire daughter, human son.

I’m assuming that you mean that offspring of some biracial matings have a chance of being the race of either of their parents. Let’s explore how this could happen. Say there is a single genomic feature that makes people vampires. It could be an extra chromosome, or an allele on a chromosome. We’ll call it V. V acts in a dominant fashion so anyone with a copy of V will become a vampire, and anyone without a copy will be a human. If a Vampire is homozygous for V, meaning they have 2 copies of it, all of that vampires haploid gametes will also contain a copy of V. This would mean that all of a vampire’s children would have a copy of V, and therefore be vampires. Not what you want. Let’s say instead the Vampire parent is heterozygous for V, so maybe they only have one copy of the V chromosome, or one copy of the dominant allele. In this case only half of the vampire’s offspring will receive a copy of V. So half will be human and half will be vampire. That’s perfect right? The problem with this is that if a vampire is only heterozygous for the V trait, then vampires won’t “breed true”. When two vampires mate there will be a 25% chance that they have a normal human child! We can’t have that! And to make matters worse, another 25% of the vampire’s couple’s spawn will receive 2 copies of V and then be homozygous for V. When those children mate with humans they will always have vampire offspring. I’ll spare the reader the trouble of going through all the other possibilities, of making V recessive or codominant or multigenic. Classical genetics isn’t going to work for us.

The central problem is that when two organisms produce offspring, half of the progeny’s genome comes from one parent, and half from another. We can fiddle with things to make what we want happen to this first generation, but when they go to reproduce, they have an equal likelihood of passing on their father’s chromosomes as they do their mother’s. We need to make some exceptions to these rules.

Perhaps you’ve heard in the news about a new technology under development called “gene drive”? The idea is that you can make an allele of a gene that is capable of “infecting” or copying itself onto another allele. So when an organism is heterozygous for a gene drive allele, the gene drive allele modifies the other allele to become a gene drive allele as well. The organisms is then homozygous at that allele. In this way the technology would allow the rapid spread of a certain allele in a wild population of organisms. It’s good for our purpose because it lets us get rid of those pesky heterozygotes. This sort of system actually already exists in nature in a couple forms. Homing endonucleases and transposons both have this sort of functionaility. Gene conversion is another way this could happen. So imagine our vampire allele V is a gene drive allele. All vampires will be homozygous for the allele and when they mate with humans all of the offspring will be heterozygous, the V allele will overwrite the human allele and all of the offspring will be vampires. Now, imagine the human counterpart to V, allele H, is also a gene drive allele. When a heterozygous embryo is created from a vampire-human cross it will contain both H and V. V will try to overwrite H and H will try to overwrite V. Say the chance of V or H winning is 50/50 and now our vampire human cross is capable of producing both vampires and humans, both of which will be homozygous.

A female of one race can be impregnated by a male of another race, but it fails with switched races or sexes, e.g. vampire mother + human father = no offspring.

This is definitely the easiest of your requests. Although it still is difficult to achieve with classical genetics there are some well-documented ways in which this could occur. The simplest is some sort of non-genetic barrier to reproduction. An example might be if human sperm is incompatible with the hostile environment of a female vampire’s reproductive system. Or if the human sperm was unable to recognize the zona pellucida of a vampire’s ovum. There are many ways in which fertilization could be blocked in one mating but still work in another. If you aren’t keen on non-genetic barriers, another option is imprinting. Imprinted genes are epigenetically silenced depending on their parent of origin. Humans have a handful of imprinted genes where expression from allele from the mother or father is silenced. If a human male silenced an important gene that a vampire male did not, then a male-human female-vampire coupling might result in a serious deficiency in that gene, which could make the embryo non-viable.

The race of certain offspring may be determined by the race of the parent with the same sex, e.g. dwarf mother + gnome father = dwarf daughter | gnome son.

This one isn’t easy either. The immediate solution that comes to mind is “Let’s put it on the sex chromosome!” and it is very wrong. Say the gnome Y chromosome contains an allele D that makes the sons who receive this Y be gnomes. Now, whenever a gnome mates with another race his sons will be gnomes and his daughters will be the race of the mother. That’s great right? The problem is, how do you ever get female gnomes?

You might think maybe we can explain this with imprinting, but imprinting only gives us a parent-of-origin effect. We need to correlate the parent-of-origin with the sex of the child, and that means putting imprinted alleles on the sex chromosomes, but I still don’t really see how it could work.

Some races are restricted to a single gender and rely on other races for reproduction, e.g. sphinx mother + centaur/minotaur father = sphinx daughter.

This is your second easiest requirement. I think the best explanation for the sphinx is that the features that make a sphinx a sphinx are in the mitochondrial genome. The mitochondria are inherited only from the mother through the egg and so they get around our issue of hybrids. A sphinx egg doesn’t care about the sex chromosome in whatever sperm it receives, it just makes a female sphinx.

Some hybrid races are infertile when mating with each other, but can reproduce with members of other races (or at least one sex can), e.g. human + elf = half-elf, human + dwarf = half-dwarf, both are considered halflings, halfling + halfling = no offspring, but halfling + human = human (or maybe halfling again).

Oh man, this is hard. Having hybrids be infertile is easy, a lot of real ones are anyways. Having them be infertile with each other, but fertile with their parent species does not make a whole lot of sense. There could again be non-genetic barriers. Say female halfling reproductive systems don’t develop quite right, but the male reproductive system works fine. A halfling-halfling pairing wouldn’t produce offspring because the female would be entirely infertile. The halfling male on the other hand could still reproduce with either of its parent races. I have to warn you though that the result of a halfling-purebreed coupling is going to be a three-quarterling and may not be what you want. If you really want to get into the complex, hard science of this sort of stuff you should also take into account Haldane’s rule.

I’ve written a lot and I haven’t gone into as much detail as I would like to explaining everything. If you have any questions or need clarification I can do my best to explain in the comments, or through chat if you’d like.

$\endgroup$
3
$\begingroup$

I used to breed Netherlands Dwarf rabbits, and one of the most difficult things is the high mortality rate of kits. See, dwarfism as an allele in rabbits is a lethal recessive trait. Let's refer to the dominant as D, and the recessive as d. A true dwarf has the genotype Dd. That gives it the short ears, small body, and disproportionately large head that makes it absolutely adorable. Rabbits with the genotype dd are referred to as "peanuts". Their digestive systems are nonfunctional, and if not destroyed, they starve within days of littering. Rabbits with the genotype DD are not dwarfs, and such offspring of true dwarfs are typically destroyed. See the attached Punnett square for breeding true dwarfs.

Half the litter is lost, and half are true dwarfs like the parents. In the example of your vampir, suppose humans have the phenotype vv, vampires are heterozygous as Vv, and a creature with two vampire genes (VV) is not viable due to whatever biological phlebotinum. Humans would always breed true, obviously. Vampire human hybrids would be randomly half human and half vampire. Vampire/vampire pairings would result in the same spread as my rabbit example. If VV is not viable, that leaves human (vv) offspring of vampires to be accounted for. I think the simplest explanation would be that a chemical in vampiric blood is toxic to human fetuses and would not allow them to be carried to term.

While this doesn't fully account for all of your demihumans, nor does it directly explain sex-linked inheritance, such lethality of various gene combinations can be tweaked to produce any sort of combination of surviving offspring you wish for various couplings.enter image description here

$\endgroup$
  • $\begingroup$ Welcome to WorldBuilding Christyn! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Jul 10 '17 at 6:44
2
$\begingroup$

A lot of this boils down to Mandelian Genetics, aside from all that hoobub about chromosomes. Before i get started, i'd like to point out that Mandelian Genetics have some flaws, but that doesn't mean they aren't usefull and it certainly doesn't mean they are "obsolete" most of what we know about genetics today is still based off of Mendels findings.

The Mandelian theory basically states that each trait is a gene "packet" called an "allele". If a minotaur that has the allele for two-legged-ness mates with a centaur who obviously has the allele for four-legged-ness, then the traits are decided by which ever allele was the dominant gene. This can be seen in humans: the "brown-eye" gene is dominant, so around 75% of the time you have a brown-eyed parent and a blue-eyed parent, the children will have brown eyes.

This law can be used to explain your Sphinx issue. As long as the genes that make a creature a Sphinx are dominant over the genes that make them not a Sphinx, the child will also be a sphinx. You may attribute the fact that all sphinx-child's are female to "sex linked genetic traits", which states that some traits are found only in offspring of a certain gender. Bees are a good example of this, as they are male or female based on diet, incubation time, and pheromones that the queen creates: the females are workers or queens(depending) and the males are always drones, they develop differently before pupating.

this can also explain this rule:

•The race of certain offspring may be determined by the race of the parent with the same sex, e.g. dwarf mother + gnome father = dwarf daughter | gnome son.

The talk of halflings and crossbreeds can be dealt with by Mendels laws also. As user6760 said, Mules, as a rule, are infertile due to inconsistency of chromosomes,. A crossbred human/elf could share a similar predicament as a mule, we just have to change things a bit to make sure they can still breed with a full human or elf. Unfortunately, to coincide with everything else, this would require the Halfling to be "given" to a certain race, without being one. for instance, a Halfling with the same number of chromosomes as a human would be female, and a halfling with the same number of chromosomes as an elf, would be male. this would ensure that halfling/halfling relations would have n o offspring, but a male Halfling/female elf relation could. Additionally, a female Halfling/male human could have children.

then comes the difficult part of male elf/female halfling or female human/male halfling ... You could brush this off and not really pay attention to it, or you could say "hey, if elves and humans can work in the first place, why not these?"-but that will throw off the entire rest of the theory.

well, that's about all I can do... I have nothing for

•A female of one race can be impregnated by a male of another race, but it fails with switched races or sexes, e.g. vampire mother + human father = no offspring.

you m ay need a chemist for that one :P.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.