From the beginning of Harry Potter, it's clear that Professor Dumbledore is a brilliant man. He understands magic so well that anything he knows or does isn't a surprise. So when Dumbledore appears to understand Parseltongue in several of the books, we thought it was simply because he's smart. However, a new theory suggests that it's actually because he's a descendant of Salazar Slytherin.
The theory, written by Reddit user Obversa, is long and detailed but explains exactly how Dumbledore and Slytherin could be related. It begins with Isolt Sayre, the founder of Ilvermorny and the daughter of Rionach Gaunt — a descendant of Slytherin. Sayre marries James Steward and the two have twins: Rionach and Martha. Though Sayre couldn't speak Parseltongue, she still carried the gene for it and passed it on to her children. Martha, a squib, eventually marries a Muggle from the Pocomtuc tribe but still carries the Parseltongue and magical genes.
Fast forward a few generations, and Kendra, Dumbledore's mother comes into the picture. In Harry Potter and the Deathly Hallows, the following is revealed: "Despite the high-necked silk gown she wore, Harry thought of Native Americans as he studied her dark eyes, high cheekbones and straight nose." This is a small detail but alludes to the possibility that Kendra is a descendant of Martha, thus also making her related to Slytherin. So, when she had Dumbledore, she too passed down the Parseltongue gene, which lets Dumbledore understand the language.
According to Pottermore, you can't learn Parseltongue; a person merely inherits the gene to understand and speak it. Based on all these clues, it looks like Dumbledore is actually related to Slytherin. It would explain why he's never opposed to Slytherin students as some professors are at Hogwarts. Unconvinced? Read the full theory below.