Basically, it's a very simple, visual representation of a theory (i.e., an operating assumption): that magic in Jo Rowling's Harry Potter books is a genetically inherited recessive trait.  In other words, Punnett Squares (Mendelian model) for the Potterverse.

Eh, What?

Punnett Squares.  You know, those things you did in biology classes when you were in school?  Made understanding how ten generations of brown-eyed brunette gene donors could produce an eleventh generation blue-eyed blonde without any mutations?  Yeah, those.  Now think about the Potterverse.  And genetics.  And magic.  Yep, I think we're getting there ...  


Well, why not?  Seriously though, with all the talk of blood and lineage in the books, it's always good to take a look at how blood (or rather, genetics) could be a factor.  Granted, most of the references in the books are actually not so much jabs at blood as they are indicative of culture clashes and class tensions.  That said, however, developing a theory for the genetic basis of magic has its own interesting features -- as much for past and future implications as for the present Potterverse timeline (post HBP).


Okay, let's see ... think about it this way.  If magic is not only genetic, but genetically recessive, then there could be something to all the elitist pureblood propaganda -- but only to the degree that certain family genes have mutated over the generations to allow for certain characteristics (such as Parseltongue).  Selective breeding, essentially.  On the other hand, if all magic is genetic then the only thing that purebloods can claim as exclusive are those selective traits that run in families; otherwise, all witches and wizards are on even genetic ground because they all have a pair of recessive magical genes that enable magical ability.

Still confused?  Example.  Draco Malfoy and Hermione Granger are both magical, but he's a pureblood and she's a muggleborn.  If magic is genetic, the result of inherited genes, the only real difference between them is that the Black/Malfoy genes that Draco inherited from his parents are the result of years of selective breeding for certain characteristics (also called mutations), whereas the magical genes Hermione inherited from her non-magical parents were passed down unchanged for an unknown number of generations.  Which would, after a fashion, make her a genetic throwback as opposed to something totally new.  Her genes are old, but hardly "muddy."  (Dusty maybe.)  Her parents are thus carriers; they each have a recessive gene for magic that is masked by the dominant non-magical gene, making them capable of producing a child with magical ability while they have none themselves.

Why You?

Why not me?  Okay, so I'm not a geneticist.  I don't even have an undergraduate degree in biology or any related field (actually, what I do have is a graduate degree in history).  But I remember my biology lessons, and I find the continual use of "blood" in the books to be misleading -- fascinating, but misleading.  The continual references to purebloods, halfbloods and mudbloods is simply a gloss over what's really going on, which is a clash between cultures and classes.  What's fascinating is how the characters relate to this idea of blood.  Some are obvious (Voldemort, Malfoy, Death Eaters) and some less obvious.  Some are downright ambivalent.  Some reject the idea; others embrace it.  But no one really disputes the idea that the hostilities are about blood rather than culture (though Dumbledore comes close).

So, in short, I find the whole situation intriguing, hence my observations and conclusions above and below.  And because I've been unable to find any simple visual examples about genetics and magic anywhere on the Net, I created my own.  It's in no way exhaustive or detailed!  It's NOT a scholarly research project full of multiple points of supporting evidence!  (And never will be, either.)  What it is, I hope, is a concise and easily understood look at an idea: that magic in the Potterverse is an inherited recessive genetic trait.  

Punnett Squares (Examples)

Key: Muggle (NN); Muggle Carrier (Nm); Magical (mm)

         Muggle = someone with no magical genes; no magical ability

        Muggle Carrier = someone with one non-magical gene and one magical gene; no magical ability observed

        Magical = someone with two magical genes; a witch or wizard


Example 1: Muggle/Muggle

Results: The children from this union can have no magical genes (baring mutation -- see next section). 

#1 N N


Example 2: Muggle/Muggle Carrier

Results: 50% chance for a completely non-magical (Muggle) child

            50% chance for a Muggle Carrier child

#2 N m


Example 3: Muggle Carrier/Muggle Carrier

Results: 25% chance for a completely non-magical (Muggle) child

            50% chance for a Muggle Carrier child

            25% chance for a Magical child

#3 N m
m Nm mm


Example 4: Muggle/Magical

Results: 100% chance for a Muggle Carrier child

#4 m m
N Nm Nm
N Nm Nm


Example 5: Muggle Carrier/Magical

Results: 50% chance for a Muggle Carrier child

            50% chance for a Magical child

#5 m m
N Nm Nm
m mm mm


Example 6: Magical/Magical

Results: 100% chance for a Magical child

#2 m m
m mm mm
m mm mm

What does this boil down to?  Examples 1 or 2 would result in Dudley Dursley; Example 3 in Hermione Granger, Lily and Petunia Evans, the Creevey brothers and Ted Tonks; Example 5 in Seamus Finnigan, Severus Snape, Dean Thomas and Tom Marvolo Riddle; and Example 6 in Harry Potter, Draco Malfoy, Nymphadora Tonks, Sirius and Regulus Black, and the Weasleys.  


Other Possibilities

And then there are the other possibilities.  In particular, how do you account for Squibs?  

Within the theory as it stands, Squibs (non-magical children born to magical parents) are the result of a spontaneous genetic mutation, a deviation in one or both magical genes that either severely impairs or negates the ability to perform magic.  Reasons for this are unknown; inbreeding is a popular and entirely plausible scenario, but other possibilities include poor health monitoring, a mother that is too young or (more likely) too old and/or other unknown inherited conditions.  

Then we come to another possibility expressed by others -- could all muggleborn magicals be the result of spontaneous natural mutations?  Nature's way of enlarging the established magical population?

Well, yes, that could be true.  In which case it would essentially mean that the theory presented here is a moot point.  

But that doesn't necessarily have to be the case -- both could be true.

How?  Consider the Creevey brothers and Justin Finch-Fletchley.  All three are muggleborn wizards; but while both Colin and Dennis are wizards, we know that some of Justin's siblings are non-magical.  While it's possible that the same spontaneous mutation could show up in multiple children of the same parents, inherited genes is a more likely scenario.  It also better explains how Dean Thomas, Severus Snape and Tom Riddle are wizards even though each has a non-magical parent.

But back to Justin.  He could be the result of inherited genes or spontaneous mutation, as we know he has non-magical siblings (he could the 25% chance or 1 in 4 from Example 3); the same could be said for Hermione, who has no siblings (and the one younger sister JKR originally planned would have remained non-magical).  So, spontaneous mutation could account for some of the muggleborn population, but only some.  There are far too many muggleborn witches and wizards for all of them to be the result of spontaneous mutations.

But Rowling has also said that "magic is a dominant and resilient gene" -- so where does that leave us?

Not so far off the mark as you might think.  Magic cannot, for obvious reasons, be a truly dominant (or expressed) gene or else there would be far more wizards/witches and far fewer Muggles.  However, the gene could still be sturdy, resilient, more resistant to change over time and (of course) dominant among its own population.  This then, is also another good indication that magical genes can be passed down through multiple generations of carriers until reaching a point where it meets another such gene and produces someone like Hermione and the Creevey brothers.

And yet, could there be that many carriers in the Muggle population?  Several people have expressed doubts as to the feasibility of such a scenario.  In all honesty, I don't know -- but I suspect there easily could be.  


Consider the current magical population of the UK as presented by Rowling, around 30,000 (if I'm not mistaken), and then think about all the unused classrooms in Hogwarts.  Clearly there was once a need for them, which likely indicates a larger student population.  Also, remember that wizarding Britain has seen two Dark Lords during the twentieth century, first Grindelwald and then Voldemort.  People died during both of these conflicts, and there doesn't seem to have been a baby boom after Voldemort's first reign of terror ended.  One reason for this, as posited by AngieJ at the Lexicon, is because the post-1981 "peace" in the wizarding world was nothing like the post-1945 "peace"; as she points out, Voldemort's body was never recovered and this cast a "shadow" over wizarding Britain.  The most obvious manifestation of this "shadow" is people's fear of his very name ("say You Know Who!").  People would thus be less likely to have large families if still fearful of another conflict.  

What this means is that the magical population of the UK has diminished, and given the reactions of many purebloods (and Ron's assertion in CoS that almost none remain completely "pure") this diminishing is nothing new.  I've long suspected that there was a conflict during the eighteenth and/or nineteenth century that started this, but there's no canonical evidence that I can find.  And given that we know little about prior population levels, it seems entirely sensible to speculate that the magical gene could be floating around in a large percentage of the Muggle population.  As for why there aren't more Muggleborns than there are currently, remember that there's only a 25% chance of two carriers producing a witch or wizard.  This would explain not only the relative number of Muggleborns, but the reason why those numbers appear to be stable.  The purebloods only think Muggleborns are becoming more prevalent because so many of the "old blood" families have been killed in wars and conflicts, and those that have survived are having fewer children. 

As for the Squibs, well, they actually could be the reason for the magical gene remaining among the Muggle population.  Born to the wizarding world but unable to completely live in it, many of these people may have chosen to enter the Muggle world.  If so, and if the genetic mutation affected only one of their magical genes, then these Squibs may have been adding the magical gene to the Muggle population.  For instance:

Squib (mw)

Squib = someone with one magical gene and a defective magical gene; someone born to two magical parents but who has little to no magical ability themselves.

Example 7: Muggle/Squib

Results: 50% chance of a Muggle Carrier child

            50% chance of a Muggle Carrier child (carrying the mutated gene)

#7 m w
N Nm Nw
N Nm Nw


Example 8: Muggle Carrier/Squib

Results: 25% chance of a Muggle Carrier child 

            25% chance of a Muggle Carrier w/mutation child

            25% chance of a magical child

            25% chance of a Squib child

#8 m w
N Nm Nw
m mm mw


Example 9: Magical/Squib

Results: 50% chance of a magical child

            50% chance of a Squib child

#9 m w
m mm mw
m mm mw


Example 10: Squib/Squib

Results: 25% chance of a magical child

            75% chance of a Squib child

#10 m w
m mm mw
w wm ww

All of these examples assume, however, that the mutation that causes a Squib affects only one gene.  If it affects both genes, these squares would look very different.  For one, there would be no way for a Squib to produce a magical child or contribute the magical gene to any progeny -- and that seems next to impossible given what we know of mutations.  Therefore there are probably two Squib mutations, one of which (as shown above) allows the magical gene to pass unremarked into the Muggle gene pool and one that doesn't.  Combine this with direct Wizard/Muggle marriages over the years (and with Muggleborns who were never trained for one reason or another) and you have the makings for a gene that remains mostly silent in the majority of the world's population.  Hardy and resilient, it just keeps getting passed down through the generations. 

Then there is the possibility that inherited magical ability also has to be "sparked off" by something, or doesn't follow a simple Mendelian model like the one I use.  If there are multiple gene codes that determine magical ability, expression and power levels, then an epigenetic model is needed.  And while interesting, I just don't have the time or the advanced knowledge to pursue it!

Selected Links

For further reading:

Magic, Genes & Pure Blood

An HP Lexicon essay that covers material similar to that presented here, and postulates a bit further.

Harry Potter and the Genetics Lesson

MSNBC article; three scientists debate using HP to teach children about genetics.

The Inheritance of Magic: What It Means to the Wizarding World

More in-depth information on genetics in HP and its ramifications.