Porphyria is a rare genetic disorder that is recessive. You must carry two porphyria genes to get it and even then, expression of it isn’t guaranteed. There are several types of porphyria (acute, cutaneous, Swedish or Acute Intermittent Porphyria – AIP, , erythropoietic porphyria (EEP), one affects the central nervous system and one affects the skin. However, it should be noted that symptoms affecting both the skin and the central nervous system isn’t uncommon. Meaning while the two are differentiated, they still crossover in symptoms.
Porphyria is a blood disorder. As red bloods cells break down, they are metabolized by the body and excreted. However, people with porphyria have a dysfunction of the metabolism of red blood cells and the often the separation of iron and oxygen carried by red blood cells despite beginning to decay, becomes problematic.
Porphyria has a variety of physical and mental symptoms, including skin sensitivity to sun, deformation of skin including the creation of lesions or build up of dead skin in crusty scab like patches. Red or brown urine is also very common. Rather uncommonly, people with porphyria can have purple, blue, or black urine as well. The reason we believe George III of England most likely suffered porphyria (most likely AIP), is because aside from having bouts of madness, the royal physician recorded that George’s urine was often purple.
For reasons, I don’t quite understand, people with porphyria are encouraged to eat diets high in protein, specifically protein in the form of meat. Protein from eggs, nuts, and other sources aren’t metabolized and used as well in the body when one has porphyria as protein gleaned from meat consumption. In other words, vegan and vegetarian lifestyles are highly discouraged when one has porphyria.
Interestingly, porphyria which is treated in modern medicine with medications and blood transfusions may also benefit from cannibalism. There was a missionary in Papua New Guinea in the late 1800s who made notes on a villager who suffered severely from an unknown disease that had left his face disfigured and made him suffer bouts of extreme madness (during one of these bouts, he cut off part of his own foot), common symptoms of porphyria, who would get better after feasts that included cannibalism.
This story provided plot inspiration for an early episode of CSI: Crime Scene Investigation in which a woman with porphyria was killing people to cannibalize them, because eating of people and drinking their blood worked better than the medications and transfusions.
As a side note, historical inbreeding of royals and among small populations like villages in Papua New Guinea made the disease more common than it probably would have been otherwise. Today, only 5 out of every 100,000 people suffer from porphyria. There is no cure and it is a painful disorder as well as causing disfigurement and mental illnesses.
Hadena James 
Acute hepatic porphyria is dominant not recessive.
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That surprises me; usually dominant gene diseases are prolific. My original research didn’t say whether it was dominant or recessive but since it is uncommon I guessed it was recessive. Obviously, I guessed wrong.
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