In canon, there are wizards with one wizard parent and one muggle parent, who aren't squibs (Snape and Riddle for two).
In canon they call “squib” the non-magic-capable child of two wizards.
In MoR, that means the child has only one copy of the recessive magic gene. (Either mommy didn’t love only daddy, or one copy of the gene got messed up somehow.) But in MoR you need to distinguish between genetic|squib (has one copy of the gene), and genealogic|squib (can’t do magic but has wizard/witch parents).
All genealogical|squibs are genetic|squibs, but wizards use the word “squib” only for the former, since wizards don’t know much about genetics, and about the magic gene in particular. They call anybody who isn’t a part of magic Britain a muggle (genealogical |muggle), even though they might actually be genetic|squibs.
An example: Wizard Nasty Pants does the nasty with lots of muggle women a couple of centuries ago. He doesn’t like commitments, so he abandons the women to raise their children alone.
All his children are genetic|squibs, but they’re raised by muggles and—after Mr. Nasty dies because he tried that with a witch married to a Gryffindor—nobody knows they had a wizard parent.
Mr. and Mrs. Ancient Robes have a squib (Mr. Robes was often away on ancient business), and Mrs. Robes leaves him to be raised by a muggle family, because she doesn’t have the heart to see him killed (Mr. Robes is kind of old-fashioned that way), and claims he died at birth or something.
A couple of generations later the magic gene still exists in a lot of Mr. Pants’ and Mrs. Robes’ muggle-raised descendants: half a genetic|squib’s children are also genetic|squibs, if the other parent is genetic|muggle, and people used to have lots of kids until recently. But they’ll be genealogic|muggles, and any wizard will call them muggles, because they’re not known to have a magic parent.
And then, two of these genetic|squib descendants marry (either the two trees intersect, or a couple of kissing cousins decide to do more than kiss), and a quarter of their kids are wizards. Magic Britain will call them muggle-born (or mudbloods, depending on political inclination), although in fact they’re lost descendants of wizards.
Similarly, when Ms. Broad Horizons, a witch of liberal inclination, falls in love with young muggle (but genetic|squib) Bendsinister McPants, half her kids will be wizards, and Magic Britain will call them half-bloods.
Since there is a single magic gene (apparently), it is also possible that a mutation will toggle between the magic and non-magic alleles. So it is also possible, though probably much rarer, that a squib (or, even less likely, a wizard) appears from completely non-magical parents, or that two magic users have a squib or non-magic child, due to simple mutation. How likely that is depends on the complexity of the gene, but it’d have to be much rarer than the above scenarios, unless there’s magical interference in mutation rates for that specific gene.
Also, we cannot completely ignore the possibility of the "magic machinery" (the one that recognize the genetic marker) to have some kind of shuffling process that'll occasionally turn on or off the magical marker when an egg is fertilized. Either randomly, or based on events (triggers like "an egg fertilized exactly at the second where the moon is the fullest will have a high probability of having the magical marked added").
We have no hints towards that, so Occam's Razor would tend to give it a low probability, but it would seem coheren...
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 81, which should be published later today. The previous thread passed 400 comments as of the time of this writing, so it will pass 500 comments soon after the next chapter is posted, if not before. I suggest refraining from commenting here until chapter 81 is posted; comment in the 12th thread until you read chapter 81. After chapter 81 is posted, I suggest all discussion of previous guesses be kept here, with links to comments in the previous thread.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.) When posted, chapter 81 should appear here.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight,nine, ten, eleven, twelve.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: