So for those who are reading my fic and don't know anything about BtVS canon:
Dawn Summers
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From Wikipedia: Dawn is first introduced as Buffy's little sister at the end of Buffy season five premiere "
Buffy vs. Dracula", despite Buffy having been previously established as an only child. Initially, the mystery of Dawn's sudden existence is not acknowledged in the series, with the other characters accepting her as a part of the status quo. Four episodes later Buffy discovers Dawn is in fact a mystical object known as the Key; a group of monks transformed the Key into human form and sent it to the Slayer for protection from the villainous Glory (a hell god, and season five's big bad). The memories of Buffy and her associates were altered, along with relevant records, so that they believed her to have always existed as Buffy's sister. When Dawn learns of her origin, she resorts to self-harm and runs away from home, until Buffy assures her they are real sisters no matter what. The season arc concerning Glory and what she wants the Key for is eventually resolved, and Dawn goes onto live as normal a life as possible with the Vampire Slayer for your big sister.
However, the exact nature of Dawn's origins was never fully explained in the show. She was made out of a glowing ball of energy by some monks, and she and Buffy 'had the same blood' or similar enough, but it was never really specified what that meant. Basically, the writers said 'Magic did it!' and moved on. Many fics have explored this plot hole, some saying that Dawn was formed solely from Buffy's DNA and is the equivalent of a magical clone, or saying that she only got one half of her chromosomes from Buffy, and is the equivalent of a magical test tube baby, and exploring the possibilities of who the other 23 chromosomes could have come for, causing wacky hijinks to ensue.
I've gone with option two for my fic, and chosen Dracula as the donor for the other half of Dawn's DNA. While it is canon in Buffy that vampires can't have children the traditional way, magic is a very convenient handwavy device for explaining things, and there's canon precedence. In the spin off show 'Angel' the character Connor was the miracle child of two vampires. While Connor had enhanced strength and abilities, he did not have fangs or a thirst for blood or an aversion to sunlight or any other qualities inherent to vampires. So it's perfectly logical (in Buffy canon) to suppose that someone who was the genetic child of a human and a vampire wouldn't show any obvious outward signs of their vampiric heritage. Especially if magic is involved in the creation of said child.
So to recap, in BtVS canon, Dawn Summers was created out of a ball of energy by a bunch of magic using monks using (at least) Buffy's DNA, and she and Buffy were given fourteen years of fake memories of being sisters, but they still love each other and consider each other as sisters despite Dawn's wacky origins.
In my fic: Dawn Summers got both Buffy's DNA, and Dracula's. Due to the monks being on a time crunch when they made Dawn (it's canon that Glory was literally busting down the door as they did the spell) they simply grabbed some of Buffy's chromosomes and some from the person nearest her at the time of the spell, which just happened to be Dracula. Dawn ran into Dracula sometime after the end of the series, he stalked her for a bit, and then he turned her when she was on the brink of death's door after being mortally wounded in a fight.
I'll do another post on characteristics of vampires as they are portrayed in the Buffy universe, specifically dealing with exceptions to the rules, such as vampires-with-souls (two in existence; Buffy dated both), Dracula (Buffy didn't date him, but they did drink each others' blood while Buffy was under Drac's thrall), and really old vamps that are incapable of passing for human (turok-hans, The Master, Kakistos, etc.).
If you have any more questions about Dawn, please drop me a line.