Alexandria's Genesis is a myth that started circulating circa 2005, although it claims to be much older. It appears to have originated from a piece of Daria fan fiction written by Cameron Aubernon in 1998 (archived section). While internet memesters have modified it to suit their needs, the core elements remain very close the the original post.
The most common version is that a meteor streaked across the sky of ancient Egypt, leaving in its wake a outbreak of babies with pale skin and purple eyes, who later turned out to age slowly, remain permanently healthy and fit, live for approximately 150 years. These people emigrated North, and were mysteriously lost until a woman named Alexandria appeared with the condition in London in 1329.
The condition is generally claimed to be a mutation, with the implication that it might appear anywhere -- if it is a recessive gene it is deeply buried (this diverges from the original post, in which AG is a dominate gene). If your child has it, they will be born with purple eyes, or their eyes will turn purple at around six months. They will have pale skin but never sunburn; they will be unusually attractive and healthy throughout their lives; they will poop less than the human norm; they will have brown hair on their head, but otherwise suffer from the curse of alopecia; and females will not menstruate at puberty, but can still bear children.
It hard to judge how many people might actually believe this myth, but it does pop up as a meme every so often. One has to expect that a certain percentage of the population doesn't care at all about truth, but do care about 'interesting facts'; these people keep the myth alive.