The Rosewood Syndrome is a genetic disease caused by a magical curse the elves placed on the demon offspring several thousand years ago, an attempt to wipe out the demons that very nearly succeeded. The elves made it so that any child born of a demon would be stunted in their magical talents and would not be able to kindle demon magic, having to rely on the two separate branches of magic (Earth & Ley Line) without the ability to mix them. The Rosewood Syndrome is passed down genetically as trait attached to the wild-type demon gene that carries the ability to perform true demon magic, and anytime a witch child is born with the ability to kindle demon magic the Rosewood genes kill them before they reach the age of 2. It appears that Rosewood Syndrome may be a sort of magically manufactured auto-immune disease, but this is not confirmed.
There have been only two survivers in the history of the disease, Rachel Morgan & Stanley (Lee) Saladan. Both survivors owe their lives to genetic tampering done by Kalamack Sr., the father of Trent Kalamack (though the fate of a third patient, Jasmine, is not disclosed). It appears that Kalamack Sr was not aware that both Rachel & Stanley are genetically able to kindle demon magic, and in fixing the mitochondria he essentially enabled two demons to grow to adulthood for the first time in 5,000 years, directly countering the elven curse.
Genetics notes & ConjectureEdit
Harrison's choice of mitochondrial inheritance for Rosewood Syndrome/demon genes was exceedingly well-chosen. Mitochondria can carry more than one copy of DNA, giving them the ability to "carry" disease by having one disease copy and one normal copy (Lemire, 2005). While mitochondrial DNA (mtDNA) is usually inherited exclusively from the mother, paternal contamination is possible and has been demonstrated (Schawrtz and Vissing, 2002).
Selection, shmelection! Usually, every child of a woman carries almost all of her mitochondrial DNA, except the ones that weren't segregated into her egg cells during gametogenesis. Nevertheless, mitochondrial DNA is not invincible to genetic drift, and natural selection works for mtDNA almost as well as for DNA that follows Mendelian inheritance. The demon genes/Rosewood Syndrome is a categorically "deleterious" trait that leaves affected individuals significantly unfit--all children born with the trait die before the age of 2, so none of them can breed and pass on the demon gene predominance in their mitochondria. Statistically, one would expect that incidence of the demon genes would become exceedingly rare by now, as witches with less and less demonic mtDNA survive their more demonic sisters. Still, the witch population carries the genes so commonly that even after 5,000 years of only being able to produce viable offspring with non-demon-dominant mitochondrial heteroplasmy, there are thousands of "Rosewood carriers" in Cincinnati's witch population. Additionally, the syndrome has colloquial acceptance: Gordian Pierce mentions that two of his three siblings died of Rosewood syndrome. Rachel should be one in a million, and witches with as many demon enzymes as Gordian Pierce (must be high, but just not enough to trigger Rosewood, if all his siblings died but he survived) should be just barely a few in a million as well.
Lemire, B. "Mitochondrial Genetics" (2005). Wormbook, ed. The C. elegans Research Community, WormBook, doi/10.1895/wormbook.1.25.1, http://www.wormbook.org
Schwartz, M., and Vissing, J. (2002). Paternal inheritance of mitochondrial DNA. N. Engl. J. Med. 347, 576–580.