The Genetics of Harry Potter

 

            In an attempt to discover how the genes of witches and wizards travel from generation to generation in the fictional work of J. K. Rowling, this writer has taken some liberties with the plots as I have observed them in the movies and as my wife has explained from her reading of the books.

 

There are few terms that need to be understood before one can look at the genetics:

 

            Muggle – noun.  Non magic folk who are born to parents who are not a witch and wizard.

 

Mudblood – noun.  Derogatory term used to describe someone who has magical powers, but was born to parents that do not.  Also, someone who is born of a family that includes both magical and non-magical persons.

 

Hermione Granger (left) was born with magical powers to parents (right, shown with Mr. Weasley (in the hat)) who were muggles.  The muggle world was intriguing to people like the Weasley’s, but were looked down upon by other families like the Malfoys who thought that only the purest bloodline families should be allowed to practice magic.

 

Squib – noun.  A person who is born to a witch and wizard but do not have magical powers of their own.  This is generally the result of mating outside of the witch/wizard bloodline.

 

Argus Filch was born into a magical family but was unable to perform magic himself.  He became the fairly unsavory caretaker of Hogwarts and earnestly sought to catch any and every student her could doing mischief.  His constant companion was his cat Mrs. Norris.

 

Half-breed – noun.  A term applied to someone who is born to parents of different species that can mate to produce viable offspring (i.e. giants and humans).  These people generally take on properties of both parents and can be magical as well as have additional powers, depending on what their parents were.

 

Rubeus Hagrd (left: father human, mother giantess) is considerably taller and stronger than most humans, but not as tall or stupid as a giant.

Filius Flitwick (center: had a great-grandparent that was a goblin, making him shorter than most humans but also more cunning and clever.

Fleur Delacour (right: had a grandparent that was a Veela, similar to the sirens of Greek mythology) giving her the ability to enchant the minds of men as well as cast her witch charms.

 

Purebloods – noun.  Witch and wizard families that have no apparent mixing of blood with people who are not magical.  Generally, anyone who violates the purity of the bloodline would be expelled from the family or disowned.  Due to the dwindling number of families that are pureblood, there has been an increased amount of inbreeding to secure the bloodline, resulting in a deterioration of the mental states of these families and predisposition to paranoid delusions and violent behaviors.  People of pureblood assume themselves to be superior to others, especially mudbloods whom they deem inferior and believe that they should not be allowed to practice magic.  If we can take the line from Harry Potter and the Chamber of Secrets seriously, it gives us some insight as to the unlikelihood that there are any true purebloods left.  “Why, there isn’t a wizard alive today that’s not half-blood or less.”

 

The Malfoy family (above, left) was a pureblood family who fiercely defended their family heritage, claiming to have no members to ever betray the blood and marry a muggle.  The Weasley family (above, right) was also pureblood, but were fascinated by muggles and their ways.  The Weasleys took in Harry when he had no place to stay and Ron Weasley eventually married Hermione granger, ending part of the pure bloodline.  Sirius Black (below, shown in front of a small part of his family tree) was expelled from his family and his picture burned from the extensive family tree when he and his cousin chose not to follow in the family ways and try to prevent muggles and mudbloods from learning magic.  He was a cousin of Narssisa Malfoy, the sister of Bellatrix Lestrange, the witch who killed Black in Harry Potter and the Order of the Phoenix.


 

For the purposes of this experiment we will assume that there are only three possible mating pairs (each ideal in nature): two muggles, a muggle with a witch or wizard and a witch with a wizard.  We will also assume that the line from Harry Potter and the Chamber of Secrets is a true statement (regardless of what the more “noble” families would have us believe) and use halfblood (hybrid for all traits) alleles for pureblood.  We will assume that the witch/wizard trait, which is a dominant trait according to the Official Harry Potter website, must also be polygenic and incompletely dominant.  This means that your average witch/wizard would have the genotype AaBbCcDdEe (halfblood) down to AaBbCcddee (again idealized to the first three allele pairs but more realistically evenly distributed throughout the genotype like AAbbCcdd, aaBbCcDd, AabbccDD, etc. and assuming, that to be a witch/wizard, the number of dominant witch/wizard alleles needs to be at least three of the ten).  The overall trait is determined as the blending of the alleles.

This also leads us to see that Hagrid’s quote is probably not correct since the first generation of witch/wizard offspring would have a greater than 50% probability of having five or more witch/wizard alleles.  The fact that there are so few purebloods is easy to describe since one would have to have all ten alleles for the witch/wizard trait to be a true pureblood.  It can also explain the relative rarity of the squib since the probability of two magical parents giving birth to a child without magical powers is slightly greater than 3%.

 

With the parents being the idealized witch/wizard (AaBbCcDdEe x AaBbCcDdEe) the following results can be obtained:

           

Phenotype

Genotype(s)

Probability*

Percentage*

True Pure Blood

AABBCCDDEE

1/1024

0.10%

"Pure Blood"

3-9 capital letters

992/1024

96.88%

Squib

0-2 capital letters

31/1024

3.03%

 

With the parents both being muggles the following results can be obtained:

 

Phenotype

Genotype(s)

Probability*

Percentage*

True Pure Blood

AABBCCDDEE

0/16

0.00%

Mud Blood

3-4 (most possible) capital letters

5/16

31.25%

Muggle

0-2 capital letters

11/16

68.75%

 

With one parent being a witch/wizard and the other being a muggle the following results can be obtained:

 

Phenotype

Genotype(s)

Probability*

Percentage*

True Pure Blood

AABBCCDDEE

0/128

0.00%

Mud Blood

3-9 capital letters

98/128

76.56%

Muggle

0-2 capital letters

30/128

23.44%

 

*The probabilities and percentages are based on the ideal or best-case scenario (even hybridization of the parent generation).

 

The Punnett Squares of the above mated pairs are available in Microsoft® Excel© format here.  Click on the tabs at the bottom of the spreadsheet to see the different mated pairs.