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The demon king Sparda had an affair with a mortal woman. This union produced a set of twins named Dante and Virgil. However, a peculiarity occurred between their forming. The one called Dante was born a half-demon, with access to demonic powers from his father. These include, among other things, super strength, speed, and immortality. The other, Virgil, was born completely human, with no trace of demonic heritage or abilities.

Identical twins develop from one zygote, which splits to form two embryos. These twins are supposed to have the same genetic origins and therefore the same DNA. How could one twin be born a hybrid with demonic abilities, while the other is born a worthless mortal?

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    $\begingroup$ So you have demons in your universe, but wonder how come the two identical twins are not completely identical? If you absolutely must have a scientific explanations, you must give up the "identical" part and make them fraternal twins, with different fathers; this motif of twins with different fathers and thus different destinies is older than the hills -- for example, Castor and Pollux, the twins after whom the Gemini constellation is named. $\endgroup$
    – AlexP
    Apr 2, 2019 at 14:39
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    $\begingroup$ The demonic powers are not genetic (or maybe they are but are latent until activated), instead they are bestowed by a blessing the demon king gave to the first born but not to the second. The thing is you're asking for a scientific answer to a non-scientific question so the answer could be anything. $\endgroup$
    – Tim B
    Apr 2, 2019 at 14:53
  • $\begingroup$ This feels way too broad to me. $\endgroup$
    – Tim B
    Apr 2, 2019 at 14:53
  • $\begingroup$ @Tim B so just switch the tags? $\endgroup$
    – Incognito
    Apr 2, 2019 at 15:12
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    $\begingroup$ @Incognito It's not about the tags - it's about the limitations on answers. Demons aren't real so the answer can be literally anything anyone can come up. Hence unless you provide some way to narrow it down and no what the better answers are it's too broad. $\endgroup$
    – Tim B
    Apr 2, 2019 at 15:14

7 Answers 7

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X-Men solution

Mutations can happen in the embryo after it has split in two.

Discworld solution

Most beautiful quote from his works:

"Some genetics are passed on via the soul."

-Sir Terence David John Pratchett

The latter has been used in Shaman King, for example, to justify why the antagonist is a very powerful being while his identical twin is only moderately more powerful than a human.

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Mono-allelic expression

Your identical twins might have the same DNA, but that doesn't mean they have to use it the same way (assuming that human and demon genetics are close enough, that we can apply 'normal' mechanisms).

Lets postulate that the activation of demonic heritage genes in hybrid species is not 'on' by default, but needs a certain activator (this is important because many species don't cope well with demon traits, who'd have thought ...).
This (potential) activator gene is both present in the human genome and in the demonic genome, however only the demonic version actually activates all other demonic genes - and one of these two is chosen at random during zygote maturation.

Such mechanisms exist in nature, the most prominent example being X-chromosome silencing in female mammals, but it can also happen to other individual genes (couldnt find a wiki link here).

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Chirality - these are not technically Identical Twins, they are Mirror Image Twins. However, since people are generally symmetrical, it is hard to tell the difference at first glance. Demonic Heritage boosts this symmetricality to make the host supernaturally more attractive, making it even harder to differentiate that these are not classical Identical Twins

In real life, this leads to differences like one twin being left-handed and the other right-handed, or their hair naturally parting to different sides (due to the placement and direction of the crown).

The demonic powers that Dante develops are somehow dependant on physical structures in his body - when mirrored in Virgil's body, these structures are inert and no abilities are observed.

"But wait! If they still have the same DNA, and these structures, how can Virgil be completely human?", your Devil may cry. The answer is, Demons are metaphysical entities - demonic heritage is not strictly a product of the DNA, but rather of the Soul. They are both just as human, and just as demonic as each other - but it can only be detected in (or passed on by) Dante.

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  • $\begingroup$ I would especially like this if Dante was the righty rather than the lefty (as is traditional with demons). $\endgroup$
    – No Name
    Apr 3, 2019 at 1:46
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Fraternal Twins

If you are willing to use fraternal-twins instead of identical-twins, this difference can explained away very easily. They would be siblings, that didn't inherit the same genes.

Dizygotic (DZ) or fraternal twins ... usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. … (they are) essentially, two ordinary siblings who happen to be born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings.

Otherwise Epigenetics:

If they have to be identical, then you will need epigenetics to explain the difference. Epigenetics basically explains that what happens to us in our life, can affect the way our bodies choose to express our genes. For example if you are susceptible to certain diseases like psoriasis, if you have a stressful childhood you may develop the disease, but if you have a stress free childhood you may never develop it (even if your later adult life is stressful).

In a famous dutch example, mothers who experienced starvation during pregnancy, caused changes in the gene expression of their children. These temporary changes to gene expression have been noted to even last several generations in some cases.

So differences in the situation both these children were raised in can affect their gene expression as adults. Assuming that the demon power gene is some aspect of the twin's body that can be activated by certain conditions during their youth (and only then), you can have one twin that has demon powers and one that doesn't.

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    $\begingroup$ This is the most realistic answer. $\endgroup$
    – lilHar
    Apr 2, 2019 at 15:58
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Identical twins are not identical. They will ALWAYS have slight differences and these differences are usually caused by variances in the physical environment. For instance, one twin gets sick while young and the other does not. If something like this were to happen during a critical stage of their development say, during the limited time when demonic powers develop, then it's feasible that one twin could have demonic powers while the other does not.

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Demons are not animals, they don't have DNA.

The heritability of demonic attributes is not governed by their genetic code. It's something else. You make up what the something else is, or just don't explain it at all. They're demons, there has not been a serious scientific inquiry into their procreation, let alone a hybrid demon-human, which shouldn't be possible anyway.

They procreate magically using their demonic powers

So the demon king procreated with the human using his demon powers to make such a union capable of producing a child, and imbued his spawn with powers, but after that, the zygote split, but there was only a single imbuement of power, so one got it and the other didn't.

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Twin to twin transfusion syndrome

twin to twin transfusion fetuses.  one big one small

When you share a placenta, it is possible for the blood supply in utero to favor one twin over the other. One winds up huge and red, the other pale and stunted.

https://en.wikipedia.org/wiki/Twin-to-twin_transfusion_syndrome

As a result of sharing a single placenta, the blood supplies of monochorionic twin fetuses can become connected, so that they share blood circulation... This state of transfusion causes the donor twin to have decreased blood volume, retarding the donor's development and growth, and also decreased urinary output, leading to a lower than normal level of amniotic fluid (becoming oligohydramnios). The blood volume of the recipient twin is increased, which can strain the fetus's heart and eventually lead to heart failure, and also higher than normal urinary output, which can lead to excess amniotic fluid (becoming polyhydramnios).

They are identical. Your demon twin gets the lions share of in utero resources. The human one gets much less But hopefully over the course of the story it will become evident that the human twin is not entirely normal either.

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