Harry Potter & Genetics- a follow-up

I hope you all enjoyed my last blog post. Since writing it, I have had some more thoughts about the mode of wizarding inheritance. I found another person’s blog post (which you can find here ) arguing that the wizarding gene is dominant, rather than recessive (as I was arguing in my last blog post) and follows the inheritance pattern of Huntington’s chorea:

‘The Huntington's Disease Collaborative Research Group (1993) proved that the disease was caused by CAG (cytosine-adenine-guanine) trinucleotide repeats. The Huntington gene is dominant and autosomal (not linked to sex chromosomes). Normally, a person has 11 to 34 CAG repeats in the gene of interest, which causes the transcription of the normal huntingtin protein. Unfortunately, when an individual has 42 to over 66 CAG repeats, the abnormal huntingtin protein transcribed causes serious symptoms later in life. The huntingtin gene with an abnormal number of repeats shows dominant patterns of inheritance over the huntingtin gene containing a normal number of repeats. Let us postulate that the gene determining magical ability contains trinucleotide repeats.’

At first I thought there’s no way of knowing which of our theories is closer to the truth without knowing the relative populations of wizards to muggles to squibs, but fortunately there is a wealth of information on the Harry Potter Wikipedia page and I think from that I can make some more progress with the problem.

J.K. Rowling has been recorded to have said that there are about 600 students at Hogwarts, meaning there are about 86 students in each of the 7 years. If we assume that all wizarding children in the UK go to Hogwarts, that means that on average, 86 baby witches/wizards are born per year. The average wizarding lifespan (according to a headline from the Dailey Prophet) reached 137¾ in the mid 1990s. If we multiply the average number of wizards and witches born per year by the average lifespan, we get the average wizarding population in the UK.

86×137¾ = ~11846 magical people in the UK

The average population in the UK in the 1990s (when the books are based) was 58.02 million. If we divide this by the wizarding population, we get the ratio of wizards to muggles, which comes out as 1:4897. This means that for every wizard, there are about 5000 muggles.

If we compare this to the occurrence of our model diseases, it should tell us which is the most accurate. The prevalence of Huntington’s chorea is 12 people per 100,000, or 1 in 8333, which is slightly too low to account for the wizarding population.

One example of an autosomal recessive disorder is Cystic Fibrosis, which is caused by a mutation in a single gene (CFTR), and so fits in with my model of how the ‘wizardry gene’ could be passed on. However, about 1 in 2500 people in the UK are born homozygous for Cystic Fibrosis, which is too high compared to the number of witches and wizards in the UK. So it looks like neither of us are correct!

However, these two diseases can never act as an accurate model for wizarding inheritance, if we take selection pressures into account. Instead of being a disease, wizarding is advantageous, and has been selected for over thousands of years. Although the life expectancy of Cystic Fibrosis is beyond the age of reproduction, so it is possible for sufferers to have children, it is very unlikely that they would procreate with another sufferer. This is the opposite for wizards/witches, who often marry other wizards and witches, especially in pure-blood families.

To truly test whether wizarding could be autosomal recessive, we would need to find a model autosomal recessive trait which is selected for (or desirable) in the population, due to some sort of advantage, and is caused by a single gene. If anyone reading this can think of one, then I am all ears!