Disclaimer: This is just a theory! A very simplistic theory! The author is a second-year Hogwarts student and not a professional at muggle genetics. The author is open to receiving owls to clarify any sections and to work together to fix any science blunders.

In our wizarding world, we have four main blood statuses: muggle-born, half-blood, pure-blood, and squib. We look at someone’s family history to recognize their blood status appropriately. Muggles take a different approach and categorize blood types using genetics. The most common is the A, B, AB, O blood type system, which is determined by a person’s genes in their DNA. How would a muggle geneticist explain our wizarding blood status?

First, let’s go over some basic concepts of genetics to keep in mind while reading this theory:
1. A gene is a section of your DNA that determines a characteristic. For example, the blood status gene would determine the characteristic of blood status.
2. Each human being has two copies of a gene: one from their mother and one from their father.
3. There can be many versions of genes, which are called alleles.
4. Different combinations of these gene versions will result in a different characteristic. We’ll investigate this much deeper as we explore each blood status.

Starting simple with muggles. If we were to look at their blood status gene, we would find a version that has no magic at all. We can call this ‘M,’ which is the Muggle version of the blood status gene. Since a person has two copies of each gene, a muggle’s blood status genes would be ‘MM.’

Also simple to explain is the pure-blood status. We would find a gene that is magical, which we can call ‘W,’ short for the Wizarding version of the blood status gene. A pure-blood would have two copies, making their blood status genes ‘WW.’

What about a half-blood? The simplest definition of a half-blood is someone with one wizarding parent and one muggle parent. I mentioned earlier that you get one copy of a gene from each parent, so a half-blood would get ‘W’ from the wizard parent and ‘M’ from the muggle parent. Combined, a half-blood would have blood status genes ‘WM.’

A Muggle version vs. a Wizarding version of the blood status gene – which one wins? We know that half-bloods can perform magic, so ‘W’ wins! We say that ‘W’ is dominant to ‘M,’ so as long as someone has one copy of ‘W,’ they are magical.

A squib is trickier to place since they come from magical families but do not have magic themselves. The key here is that squibs are very rare, so clearly there is another gene version that is at play here. New versions come up from random changes in DNA that happen over time. The first squib, by chance, must have had a ‘W’ gene version that changed into the ‘S’ version, which is short for the Squib blood status. If a Squib came from one magical and one muggle parent, their genes would be ‘SM.’ If they came from two magical parents, their blood status genes would be ‘SW.’

Again, it’s weird to see one person have both ‘S’ and ‘W’ versions since they are contradictory. But the hallmark of a squib is that they have no magic, so ‘S’ wins! ‘S’ is dominant to ‘W,’ so as long as someone has one copy of ‘S,’ they are a squib and have no magic.

Muggle-borns have two muggle parents but have magic. To put it simply, they are the opposite of squibs. Yet, muggle-borns are theorized to be descendants of squibs. The ‘S’ gene was passed down from generation to generation as squibs integrated with muggles. But just as ‘W’ changed to ‘S,’ ‘S’ by chance changes back to ‘W’ and the result is a child that now has magic. This means that muggle-borns have blood status genes ‘WM.’ This is like a half-blood, except the ‘W’ came from an ‘S.’