Why are there so few Parselmouths?

Question

JK Rowling has stated that Parseltongue is different from other languages. You can learn to understand it (like Dumbledore) and even mimick it to some degree (like Ron did), but actually knowing Parseltongue comes from one thing and one thing only: being a direct descendant of Salazar Slytherin. More precisely:

This is a weird ability passed down through the Slytherin blood line.

This, coupled with the scenes in the Gaunt house in Half-Blood Prince, would seem to imply that all direct descendants of Salazar Slytherin instinctively know Parseltongue.

But Slytherin lived over a thousand years ago, and simple genealogy will tell you that that means his direct descendants will, by the end of the 20th century, number at the very least several thousand. Probably more. The majority of (non-Muggle-born) British wizardkind would probably be able to trace their lineage back to Salazar Slytherin in a direct line, somehow or other. And yet we seem to hear only of the Gaunts and Voldemort (plus by extension Harry) at the time the books take place.

How is Parseltongue passed on, exactly? Is it perhaps only passed on—at least as an active ability (speaking)—to eldest sons?¹

How can a thousand years of genealogy possibly produce only one single, small family of descendants? Has canon/JKR ever addressed this at all?

 

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^{1 Remember, we never hear Merope uttering a single word of Parseltongue, though obviously she understood it—she may not have been able to actually speak it like her father and brother. She was still able to pass it on to Voldemort, though, which kind of undermines the limiting power of restrictions like claiming only first-born sons inherited the trait—and also precludes any theory that Slytherin descendants had to actively choose to pass on or instruct their children in Parseltongue: Voldemort never knew his mother and was still able to speak it very well.}

Answer 1

The wizarding community is a small pool, and Parseltongue is a rarely-used skill.

If Slytherin's descendents were marrying into the general community, then you would expect them to grow in number over the last thousand years. But that may not be the case; remember that:

• Slytherin's last known descendants are the Gaunt family. When Dumbledore introduces Harry to them via a memory in Half-Blood Prince, he says:

  “Marvolo, his son Morfin and his daughter Merope were the last of the Gaunts, a very ancient wizarding family noted for a vein of instability that flourished through the generations due to their habit of marrying their own cousins.”

  — Half-Blood Prince, chapter 10 (The House of Gaunt)

• There's a strong sense of class in the pure-blood families, and they prefer to marry into other pure-blood families. That puts a tight cap on the pool of people you can marry, and you end up in a tight-knit circle (look at the Blacks, Lestranges and Malfoys, for example).

  Combine with Muggle scepticism from Slytherin himself, it's easy to see how his descendents might not have spread as widely as if they were marrying into the general population.

• The Gaunts are a particularly vicious and violent family. It's plausible that other branches of Slytherin's family tree were similarly dangerous. Even if they were looking to marry outside the family, they may not find many takers.

• The pure-blood families have been dwindling steadily, because there's such a small pool of possible partners. Depending on the degree of pure-blood fanatacism, there may be branches which chose to die out entirely rather than pollute the bloodline.

That pattern all means that Slytherin's reach within the general magical community is probably smaller than you might think.

Consider further that Parseltongue is a rarely-used skill, and typically associated with dark magic. This means that:

• It's probably only going to be spoken and discussed in select circles, not the general population.
• People may go their entire lives without realising they can speak Parseltongue. You don't encounter many opportunities to speak to snakes, and there'd be no reason to check if you had that ability.
• If somebody discovered they were a Parselmouth, they might try to hide it to avoid shunning by the wider community. Look at the reaction Harry got when he let his ability slip – more experienced magical folk would keep it under wraps.

All those factors would serve to make the pool of Parselmouths appear smaller than it actually is.

Answer 2

Other than Herpo the Foul and Paracelsus (who could themselves have been ancestors/descendants of Slytherin), all known Parselmouths are indeed direct descendants of Slytherin.

We also know that by the time of the HP series, Voldemort is the last surviving descendant of Slytherin. So we don't need to mess around talking about eldest sons: even if all descendants of Slytherin had the ability, it would still be as rare as we've seen in the modern day.

As to why Slytherin has so few descendants, I'm not aware of any canon info on this, but we can speculate.

• Given the intense desire of many Slytherins (descendants of the man himself probably more so than others) to 'keep the blood pure', and the rarity of pure-blood families, they must have had little choice in finding partners. Perhaps they were marrying each other a lot, cousins and so on, which would drastically cut the number of descendants.
  Edit: this is supported by this article ("over time their gene pool became unstable due to a habit of inbreeding"), but the HP wikia is said to be an unreliable source.

• Many of them must have turned to Dark magic, and this probably led to a lot of them being locked up in Azkaban. Pairing opportunities in prison are limited, and so a lot of the branches of Slytherin's family tree may well have ended that way.

• It's possible that some of them didn't want to have more than one or two kids. Familiarity breeds contempt, as they say. Perhaps they actively tried to keep 'being a descendant of Slytherin' a rare and precious quality.

• It's also possible that their magical ability quickly dwindled among siblings. We see this with Morfin and Merope: the former seemed much more magically able, while his younger sister was referred to as almost a Squib by their father. This could be a consequence of my first point: too much in-breeding can cause all sorts of genetic problems.

Answer 3

Assume the gene for Parseltongue is dominant. From a genetic standpoint, it will only be passed down on average to half of Slytherin's descendants (other than those who get two copies of the gene through interbreeding). If the number of children per generation of Slytherin descendants averages about two, there would be no increase in the number of Parselmouths no matter how many generations were involved. As there is a tendency in aristocratic families to have a low number of children ("an heir and a spare"), this is not an extraordinary possibility.

For this ability would be passed to all of his descendants, it would require some mechanism by which the sperm and ova that do not have the Parseltongue gene are actively suppressed from being used (perhaps that meiosis selects against them somehow?). I suppose some sort of magic sieve like this could be in place, but absent textev explicitly saying that "all" of Slytherin's descendants have this ability (a far stronger statement than that all Parselmouths are descendants of Slythern) if not some explanation of how it's possible for that to be.