In one of the most classic scenes in "The Princess Bride," the sword fighter Inigo Montoya, played by Mandy Patinkin, monologues to the Dread Pirate Roberts (Cary Elwes) about why he is so devoted to fighting: A man with six fingers killed his father and he needs revenge.
“I will go up to the six-fingered man and say, ‘Hello. My name is Inigo Montoya. You killed my father. Prepare to die.’”
- Inigo Montoya, “The Princess Bride”
But if Montoya's ambition had not been so great, if the six-fingered man had survived the encounter at the end of the film, his kids wouldn't necessarily have had six fingers, despite misconceptions to the contrary.
The idea comes from the science of genetics. Each gene, passed down from parent to child, may or may not show itself in different ways, and having the dominant version of that gene makes that version more likely to show up. For instance, the "brown eye" version of the eye color gene is considered dominant over the "blue eye" version.
The claim that having more than five fingers is actually the dominant version of the gene responsible for how many digits we have is one of those fun facts posted on Reddit threads and TikToks using the content from those Reddit threads. It’s appeared in high school biology homework, judging by some of the Google search results for “is six fingers dominant or recessive?”
However, the human body is an incredibly complex system, and the answer isn't a simple yes or no. Just because a trait is inherited dominantly doesn't mean that humans are really "supposed to have six fingers," which is how non-scientists often interpret it. Let’s review the basics of genetics first to understand what a dominant trait actually is before diving into the specifics of the sixth digit.
Mitosis vs. Meiosis: How Cells Divide
The entire field of genetics is based on exploring the instructions that an organism uses to build itself from the ground up and function every day. In humans (and most other living things), these instructions are stored in DNA. In humans, the complete set is divided into 23 chromosomes. One of these is the sex chromosome, named because it determines a person's gender at birth, while the other 22 are called the autosomes and assigned numbers.
But each cell contains 46 chromosomes — two copies of chromosome one, two copies of chromosome two, and so on. When a cell duplicates, all the DNA is copied, giving each of the daughter cells a full copy of all 46 chromosomes. This duplication process, called mitosis, is how we go from a single cell at conception to the vast size of the human body.
But there is a second kind of cell division. Meiosis produces four special cells, called gametes, instead of two, and each of the gametes contains only one copy of each chromosome. Meiosis is the process used to produce sperm and egg cells, which will combine into a single cell called a zygote during fertilization. One set of chromosomes comes from each parent — the egg from mom and the sperm from dad.
Variations of Genes Are Called 'Alleles'
But when cells in a newly fertilized zygote use DNA instructions to start “building” the human body, how do they know which copy of the chromosomes to use? There could be differences between the two sets of instructions. Each different variation of a gene is called an allele, and there's no guarantee that both parents gave the same allele to their child.
If the two copies have the same allele, you are “homozygous” for that particular trait. But if the alleles are different, you are “heterozygous,” and depending on the gene, a few different things can happen. Most commonly, one of the alleles overpowers the other.
Let's return to the example of eye color, considering just brown and blue eyes to keep things as simple as possible. As with the case of six fingers, it's a lot more complicated. In this trait, the allele doing the overpowering, which is called the dominant allele, codes for brown eyes. The recessive allele, which is the one that stays hidden, codes for blue eyes.
In this system of inheritance, the difference between having brown eyes and blue eyes is as simple as having at least one copy of the brown eye allele. It doesn't matter whether you have two copies (homozygous) or just one (heterozygous), as long as one of the copies codes for brown eyes, you will have brown eyes. So how do people have blue eyes then? You just need to inherit the blue-eyed allele from both parents instead of just one.
So to a geneticist, what “six fingers is a dominant trait” actually means is that having just one copy of the six-fingers allele makes humans have six digits, while having five digits on each limb needs two copies of the recessive allele.
Polydactyly — Being Born with More than 5 Fingers on a Hand — Is Common
We admit that it's surprising, but there's some amount of truth to the claim. From a genetic perspective, polydactyly, the medical name given to the condition, is quite common.
“Polydactyly is the presence of one or more extra digits,” said Kiely James, an assistant professor of Pathology at the University of California San Diego, via Zoom. “And that can be on the hands or the feet, on one limb or more than one limb.”
The Boston Children’s Hospital noted that somewhere between 1 in 500 and 1 in 1,000 children are born with some sort of polydactyly, whether that’s a complete sixth finger or toe or a smaller, less developed addition.
James explained that there are two main types of polydactyly: syndromic and non-syndromic, depending on whether the patient has additional symptoms beyond the extra digit or not. Here's the nuance: The answer to the question “is polydactyly a dominant trait” depends on what type of polydactyly you’re talking about.
Most cases of non-syndromic polydactyly (again, an extra digit with no additional symptoms), are inherited dominantly. Syndromic polydactyly, on the other hand, can be caused by a variety of genetic variations.
“There are actually several hundred types of syndromic polydactyly,” James, the UCSD professor, said. “That essentially means that there is one genetic change that is causing many different abnormalities or differences in the body."
She added that the conditions that cause syndromic polydactyly have many different methods of inheritance, some of which are dominant (like brown eyes), while others are recessive (like blue eyes).
But if certain types of polydactyly are dominant, why don't most people have six fingers? To put it simply, dominant inheritance doesn't necessarily mean that the dominant gene is always going to be the most common. Some of these disorders, like achondroplasia (the most common form of dwarfism), might make it harder to survive and reproduce, passing down the traits. James said that some conditions involve both syndromic polydactyly and infertility too.