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Rainbow Dash: It's a family trait. My dad had a rainbow mane, and my grandma, and great-grandmother. Every firstborn of my whole family has them.
Twilight Sparkle: Just the firstborn? Uh Dash, that's... not normal.
Twilight Sparkle: Just the firstborn? Uh Dash, that's... not normal.
Inheritance is complicated. In real life, which traits are inherited from a parent is determined partly by genetics and partly by a lot of other factors that influence how those genes are expressed. Some traits are recessive and only show up rarely, some are dominant and show up more often than not. It's a great big mess that would make Gregor Mendel tear his hair out.
In fictionland, however, this complicated and to-a-degree random system can be simplified. Here, it can happen that some specific and important trait only passes to one specific child in each generation. Maybe it's the firstborn who gets the honors (which in real life is known as Primogeniture), maybe it's only the firstborn boy, or girl, or only one in a set of twins, or any other way that the magic decides to be picky about it. Whatever the requirements are, this particular hereditary trait will only be passed to a single person, who again will pass it to a single one of their children, and so on.
What is passed along varies depending on the story. It might be superpowers or other special blessings, but it is just as often some kind of nasty Hereditary Curse. As a middle ground between them, the trait inherited might be just a particularly unusual but otherwise benign family trait, like hair or eye colour, or a fancy birthmark.
Narratively speaking, the benefit of this trope is that whatever this special trait is remains contained. If it is simply hereditary in the usual way, even if it's randomly inherited and not a case of everyone in the family getting it, it would be hard to explain why there aren't dozens of these people running around after a few generations. By having the trait only be inherited by a single person each generation, it effectively enforces a downplayed Single Line of Descent, since anyone without the trait would be fairly irrelevant to the story. In some cases, the passing on of the trait might even be lethal to the person who's passing it along, meaning that the power can literally only exist in a single person at a time.
This may be a reason why supernatural entities demand a Baby as Payment. Compare Magical Seventh Son, where the child's place in the birth order, regardless of inheritance, is significant, and Gender-Restricted Ability, where everyone of a specific gender inherits the magic. It can be a result of Recessive Super Genes.
Examples:
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Anime & Manga
- Downplayed with the Kurosaki siblings in Bleach. All three of them are the children of a retired Captain level Shinigami and an Echt Quincy. Ichigo, the eldest son, is a full-blown Heinz Hybrid with natural access to every single power type in the setting. His younger sisters Karin and Yuzu only have the ability to see ghosts (and in Yuzu's case it's so weak that she only sees them as a slight blur in the air).
- Cardcaptor Sakura: Downplayed. Early in the series, it's revealed that both Sakura and Touya have magical potential, though the best Touya can do is see ghosts. It's later revealed that their father, Fujitaka, is one-half of the reincarnation of Clow Reed. While Fujitaka also has magical potential, it's Sakura who has the bulk of it, as Clow Reed predicted.
- JoJolion: The Higashikata family has the Hereditary Curse variant of this trope, a hereditary disease that strikes the firstborn members of the family. It affects male children faster than females but can be warded off by raising boys as girls.
Fan Works
- Elementals of Harmony: Planetary Dynasts' powers only pass to a daughter when they want it to, but they can only do it once, because it means abdicating both their role and their life.
- Not Another Alicorn
: Rainbow Dash casually drops that her rainbow hair is a hereditary trait that's only passed along to the firstborn in any generation, which Twilight tries to point out is insanely unlikely but is ignored. It later turns out that this is because of a botched attempt at necromancy some thousand years ago that resulted in the soul of the rainbow-haired Princess Aurora being carried along by her descendants, causing them to inherit some of her physical traits. Rainbow Dash carries an especially Uncanny Family Resemblance because she's fully Aurora's reincarnation, while all her ancestors simply had her soul along for the ride.
Films — Live-Action
- Secret Society of Second-Born Royals: As the title implies, superpowers are passed to the secondborn child in the family.
Literature
- The Belgariad: The Royal Blood of Riva passes to the firstborn son as far as The Prophecy is concerned. They often have only one child per generation thanks to the Prophecy's meddling, but when Polgara the Sorceress takes the family into hiding for a millennium, she takes care to get each son Happily Married as soon as possible since the bloodline passes "with or without benefit of clergy".
- Bras and Broomsticks: Only one child can get magical powers. In the beginning, it was Miri who got these powers from her mother but Miri eventually gave these powers to her elder sister Rachel.
- The Dresden Files: The Archive's power is passed from mother to daughter when the mother dies.
- Gleams of Aeterna: The Elemental Lords' mysterious Fisher King powers are passed on from father to his eldest living son the moment the father dies. If the father has no sons, they instead go to his oldest surviving brother (or his sons), then to his grandfather's brother, and so on, all the way back to one of the four Creator Gods that their bloodline is descended from. Furthermore, only biological descent is considered, without regard to legitimacy and primogeniture of the child, meaning that the powers often reside in a different branch of the family than the official title, because a past Lord's firstborn son was deemed illegitimate by mortals.
- Old Kingdom: The bloodlines of the seven Bright Shiners have continued through the millennia since they were founded. The line that "sees all in frozen water" includes all of the Clayr in their glacier home, but the ones "who keep the Dead down" have a Rule of Two, the Abhorsen and an Abhorsen-in-waiting. Sabriel assumes that the next Abhorsen-in-waiting will be her son, Sam, but he feels totally unsuited to it; the Book of the Dead horrifies him to the point where he can't stand to even open it. Turns out the new Abhorsen-in-waiting is actually the half-sister Sabriel didn't know she had, who takes to the seven bells like a duck to water.
Live-Action TV
- Supergirl (2015): The powers of The Dreamer are only passed down to one female in each generation of the Nal family (though it sometimes skips a generation). Mauve was long expected to be the one to inherit the powers because her only sister Nia was born male, and thus has trained all her life to interpret dreams. When Nia starts developing powers instead, she hides them from her family, hoping to somehow be able to transfer them to Mauve so as not to break her heart, but to no avail. As her mother dies, she realizes the truth and reassures Nia she was always destined to be the Dreamer, while the otherwise accepting Mauve lashes out, saying it's unfair because Nia is "not even a real woman". In the far future, those powers are inherited by Nura Nal, a member of the Legion of Superheroes.
- Wizards of Waverly Place: Enforced. By custom among the human magical community, only one kid in a family unit can keep their magic powers as adults. Any siblings will lose their magic if they lose the Wizard Tournament. This results in some rather toxic rivalries among siblings as they try to one-up each other to be the one that gets to keep their magic.
Tabletop Games
- This is how the Legacy powerset works in Sentinels of the Multiverse. The firstborn child of each Legacy inherits all their parent's powers and develops a new one of their own, while any other children are ordinary humans. This is part of what caused the seventh Legacy's Face–Heel Turn in the Iron Legacy timeline: when his daughter died at the hands of Baron Blade he knew that he would be the last Legacy and became obsessed with cleansing the world of threats before his death, only to become a monster worse than any of those he hunted.
Video Games
- In Fire Emblem: Radiant Dawn the Apostles have the ability to hear the voice of the Goddess who is actually Yune, Goddess of Chaos, not Ashera, the Goddess of Order worshipped in Begnion, and use this ability to guide their people and justify their position as Empresses of the Theocratic Begnion. This ability is passed down matrilineally, with only the firstborn daughter in each generation able to hear it. Unfortunately for the current apostle, Sanaki, she is in fact the younger sister of the Protagonist, Micaiah, whose well-established gifts of foresight actually come from the Goddess Yune. The Dark Secret of this power is that it comes from the Apostle being Branded, which was established in Fire Emblem: Path of Radiance as being very capricious in who inherits it once a bloodline has become mixed.
- Nasuverse:
- This is one of the things that makes mage society so screwed up. The most valuable part of a mage lineage is the magical crest passed down through the family. This consists of extra magic circuits, added to with each new generation, already carved with powerful and unique spells that the family considers valuable. But there's only one crest, meaning that there can only be one heir; anyone besides the heir is left with nothing but the magic circuits they are born with, which are orders of magnitude less. Therefore, there is a lot of eugenics, backstabbing, and straight-up murder around the inheritance of these crests.
- Fate/Zero: Much of the plot kicks off because Tokiomi Tohsaka sent his younger daughter to the Matoi family (who lacked a suitable heir) since he couldn't have two heirs. The fact that he genuinely does not understand why selling his daughter to the people with rape magic might be a bad thing utterly destroyed his family in the short term, including getting him and his wife killed, and in the long term severely scarred both of his daughters and almost destroys the world in some timelines.
- Lord El-Melloi II Case Files: Waver Velvet is adopted by the El Melloi family and given most of the rights of the family head solely because he might be able to help repair the family's mystic crest, really demonstrating how valuable these things are. Several of his cases also revolve around magecraft inheritance, and it's clear in more than one instance that being the heir to a magical sociopath in a society that encourages parents to see children as tools is not a good thing.
- Fate/Grand Order: The "Grand Order" turns out to refer to the highest ideal of mage society, protecting the magic crests at all costs. The crests were invented and disseminated by the Big Bad Goetia, as part of his plan to destroy the human race. He made the crests powerful and valuable to ensure that they would survive to the modern day. Considering Goetia's personality, all the terrible side effects this policy caused were undoubtedly completely intentional.
Webcomics
- Gunnerkrigg Court: Justified with the etheric abilities Antimony inherited from her mother along with her appearance. Chapter 31 reveals that the elemental spirit in their ancestry is a "flame" that passes from mother to daughter, slowly killing the mother in the process. Even if the mother was strong enough to have another child before she died, there'd be nothing to pass on.
Western Animation
- The Life and Times of Juniper Lee: The title character is just the latest in a long line of magical inheritance. Every generation or so, a Te Xuan Xe is born to her family, meaning that they are able to see monsters Invisible to Normals and try and keep the peace in society. Juniper has a little brother but he's unable to do the same (although he frequently wishes he could — and it would be possible if Juniper wasn't born).
