Purple Orchids and Muggleborn Wizards: A Theory of Wizard Genetics

by T Brightwater

A working theory of magical inheritance should be able to account for both Squibs (non-magical offspring of magical parents) and Muggleborn wizards (magical offspring of non-magical parents) since both appear in the Potterverse.

At first glance, it would seem impossible to account for both of these occurrences by the normal Mendelian rules of inheritance. If magic is the result of a dominant gene, Squibs are easy to explain but Muggleborns are not; if the gene is recessive, the reverse is true. However, if two genes are involved, the solution becomes much easier.

A real-world example of a two-gene complex can be found in orchids of the genus Cattleya. White flowers are a recessive trait, but sometimes the offspring of two white-flowered parents have purple flowers. This was discovered by the English scientist C.C. Hurst in the late 19th and early 20th century. Simply stated, two genes, both dominant, are required to produce the purple pigment; if a plant is missing either of them it will have white flowers. However, if a white-flowered Cattleya which is missing one gene is crossed with a white-flowered one which is missing the other gene, the progeny may have purple flowers!

If wizard ability is a product of two genes rather than just one, then both Squibs and Muggleborns can be accounted for by the Mendelian “laws,” without having to posit an unusual percentage of mutations or a distressing number of extramarital affairs by wizards.

Each gene has two forms, or alleles:

A – magical gene 1 (dominant)
a – non-magical (recessive)

B – magical gene 2 (dominant)
b – non-magical (recessive)

To be magical, a person must have at least one of each of the dominant alleles. A non-magical person may either have two recessive a alleles, or two recessive b alleles.

These combinations will be magical:


These combinations will be non-magical:


Presumably “pure-bloods” would have the AABB configuration, and thus all of their offspring would be magical, even if the other parent was aabb.

Squibs could be produced by any two wizard parents who have at least one recessive allele of the same gene:

AaBB x AaBB could produce

AABB (1/4 chance) = Wizard
AaBB (1/2 chance) = Wizard
aaBB ( 1/4 chance) = Squib

AABb x AABb could produce

AABB (1/4) = Wizard
AABb (1/2) = Wizard
AAbb (1/4) = Squib

However, the cross AABb x AaBB would produce all magical offspring, since only one recessive allele of each gene could be inherited by any offspring:

AABB (1/4) = Wizard
AABb (1/4) = Wizard
AaBB (1/4) = Wizard
AaBb (1/4) = Wizard

The cross likely to produce the most Squibs is, obviously, AaBb x AaBb, in which the offspring have a 7 in 16 chance of being non-magical.

Now, looking from the other direction, it’s easy to see how Muggleborn wizards happen. If one parent has two a alleles and at least one B allele, while the other parent has two b alleles and at least one A allele, they could produce wizard offspring:

AAbb x aaBB, for example, would produce all wizard offspring of the type AaBb.

Aabb x aaBb would produce wizard offspring in the proportion of 1 in 4:

AaBb = Wizard
Aabb = Muggle
aaBb = Muggle
aabb = Muggle

Aabb x aaBB or AAbb x aabB would have a 1 in 2 chance of having wizard offspring.

This, for example, solves the problem of the abandoned character Mafalda — the witch daughter of a Squib (If Molly’s second cousin was, for example, a bb type Squib and his wife had at least one B allele, they could produce magical offspring).

Do pure-bloods have an advantage?

In one way, the answer is yes; as mentioned above, an AABB wizard’s offspring would all be wizards. However, as the excellent essay Another View of Magic, Genes and Pure Blood by Professor Koniphorus Swamp points out, Voldemort’s methods would be likely to end in the destruction of the very population he claims to be encouraging.

Often in nature there is an advantage to be had from being heterozygous — having one each of the dominant/recessive pair. The classic example of this is the gene which causes sickle-cell anemia. Individuals who have two of these genes get this blood disease; individuals who have none of them, however, are more susceptible to malaria. The Aa, or heterozygous, individual has a better chance of surviving. (See this link for an excellent discussion of this topic.) There is an evolutionary “trade-off” which accounts for this gene remaining in the population in areas where malaria is prevalent.

Since we don’t know the exact genetics of magic, and don’t have a complete genetic profile for any of the characters, this part is pure speculation, but it is at least possible that possessing one or more of the recessive genes may be an advantage. This condition is well-known in plant and animal breeding; it is called hybrid vigor.

Probably the best example of this is Tom Riddle himself. His mother, pure-blooded Merope Gaunt, seems to have been rather deficient in magical power, and his father was a Muggle. If pure-bloodedness were an advantage, one would expect the offspring of a weak witch and a Muggle to have even less power than the pure-blood parent, but Tom is apparently far more powerful than his mother was. It would be ironic indeed if Lord Voldemort owes his considerable power to hybrid vigor!

Magical ability seems to be quite a complex thing, and may depend partially on factors that are not confined to wizards. Hermione, for example, is highly intelligent apart from her magical abilities; it is possible that Crabbe and Goyle are potentially powerful but are too stupid to learn or remember spells. However, it is at least plain that pure-bloodedness by itself is not necessarily an advantage; we see many pure-blooded witches and wizards who are not as good at magic as Muggleborns or half-bloods, including Neville Longbottom, Crabbe and Goyle, and especially Merope Gaunt. However, it is at least plain that pure-bloodedness by itself is not an advantage, or Muggleborn Hermione would not be the best witch in her year.

We know of three half-bloods other than Tom Riddle who have one wizard and one Muggle parent: Severus Snape, Seamus Finnegan and Dean Thomas. Seamus and Dean seem to be about average, but Snape is certainly a skilled and powerful wizard (whatever side he’s on.)

Also interesting are the two people whom we know have one pure-blood and one Muggleborn parent. Nymphadora Tonks is a Metamorphmagus, a very rare genetic condition (Ted Tonks was a Muggleborn wizard, not a Muggle — see OotP chapters 3 and 6). Harry was able to fly well the first time he got on a broomstick, which from Professor McGonagall’s comments seems also to be very unusual. Again, this is a very small sample, but is it possible that there are anomalous abilities which are more common among those who have some Muggle ancestry?

As in the case of the sickle-cell gene, there is a trade-off; if these anomalous abilities do depend on Muggle genes, the price paid for the presence of these abilities is the occasional occurrence of Squibs. However, if Squibs marry into the Muggle population, they may contribute magical genes to their offspring, and magical ability may resurface generations later in their descendants, perhaps combined with advantageous genetic factors from the Muggle gene pool.

As in populations in the real world, variability in the gene pool is likely to be an advantage to the Wizard population as a whole. Thus, Lord Voldemort is not only evil, he’s wrong.