“A Squib is almost the opposite of a Muggle-born wizard: he or she is a non-magical person born to at least one magical parent. Squibs are rare; magic is a dominant and resilient gene.” – J.K. Rowling
- Magical people (those able to perform and perceive magic)
- Pure/half-blood- a magical person with at least one magical parent
- Muggle-born- a magical person born to two non-magical parents
- Non-magical people (those unable to perform or perceive magic)
- Squib- a non-magical person with at least one magical parent
- Muggle- a non-magical person with non-magical parents (aka probably you, unfortunately)
How can genetics explain whether an individual is able to perform magic?
We learn basic Mendelian genetics in elementary school. One of the classic examples used involves pea plant flower colors. Say pea plants can have either white or purple flowers. Each plant has a pair of genes that determine the color of its flowers. The purple gene is dominant (R) and the white is recessive (r). A pea plant with two purple genes (RR) will have purple flowers, and a pea plant with two white genes (rr) would have white flowers. If a pea plant has a purple gene and a white gene (Rr), the pea plant will have purple flowers, as the purple gene is dominant over the recessive white gene.
Let’s say that magic follows this idea. J.K. Rowling says that the magic gene is dominant. We can designate the gene as M (magic) or m (non-magic). Thus, two magical parents, each with an Mm pairing, could have a non-magical (mm) child- a Squib- in Mendelian genetics.
But where would Muggle-borns come from? It would be impossible for two non-magical parents (mm) to pass on the dominant magical gene (M). Did J.K. make a mistake?
Actually, a possible answer was proposed in by Andrea Klenotiz: “Magical ability could be explained by a single autosomal dominant gene if it is caused by an expansion of trinucleotide repeats with non-Mendelian ratios of inheritance.”
You should read the full paper if you’re interested, but the short version is that having magic could be a result of having a certain number of repeated trinucleotides (three of the possible nucleotide bases in DNA). If a person has too few repeats, they don’t have magical abilities. This means that a person could have the dominant wizarding gene, but could still lack the ability to do magic because they don’t have the necessary number of times for the gene to repeat. However, DNA replication errors are quite common. If a parent already has the dominant magic gene and a high count of repeated sequences just shy of the minimum needed to express magical abilities, it is possible that a mutation allows for accidental additions of sequences in their child. If there are enough additions, the child could have the ability to perform magic.
There’s an alternative possibility to explain magical genetics involving epistasis, or the interaction of genes. As detailed in a blog at Thoughtful Spurts, perhaps the ability to perform magic relies on three separate genes: a magic perception gene, a magic performance gene, and an activation gene. The perception gene determines if someone can view the magic world. The performance gene determines if someone can actually do magic. The activation gene determines if the perception and performance genes are expressed. So even if the person in question has the dominant gene that would allow them to perform magic, without the activation gene the magic would lie dormant.
Thus, a Muggle-born could inherit the right combination of genes from one non-magical parent with the magic gene but without the activation gene and one non-magical parent without the magic gene but with the activation gene.
Of course, there could be an even simpler explanation: it’s magic, witches.
Keep calm and science on.