Do our genes make vegetables taste bitter?
An undergraduate from California asks:
“I took a physical anthropology class in college and thought the professor told us two examples of “genetic polymorphism” are: #1, some people taste cruciferous veggies as bitter and some don’t, #2, some people don’t have a platysma muscle. Are these two things true and can you tell me what “genetic polymorphism” means? Thanks!”
The words scientists use when talking about genetics are often so unfamiliar to us that they sound like a foreign language. It is no wonder that the field of Genetics can be so intimidating.
The term “Genetic Polymorphism” is just a fancy way of saying how our DNA is different from each other. Simply put, it is what makes my DNA different from your DNA. The polymorphisms are the differences that make each of us unique.
So let’s get right to the examples that you heard about in your class. I’ve never heard about the platysma (neck) muscle example – though that doesn’t necessarily mean it isn’t true. But bitter taste is a great example!
So is a change in DNA the reason why some people can taste cruciferous veggies as bitter while other people cannot? The answer is yes! Your anthropology professor was absolutely right!
What are “cruciferous” veggies, you ask? You may not have known it at the time, but you have almost certainly seen them on your dinner table. Broccoli, bok choy, cabbage, and brussel sprouts all fall into this family of vegetables.
Their bitter taste is due to two substances with very long, hard to pronounce names: phenylthiocarbamide and 6-n-propylthiouracil. Some fruits, like grapefruit and melon, also contain these substances.
We are able to taste the flavors in our food through receptors in our taste buds. There are nearly 10,000 taste buds on our tongue's surface! Not all of our taste receptors are the same. Different receptors can pick out the different flavors of bitter, sweet, salty, and sour in our food.
How does our tongue do this? The receptors for bitter, sweet, salty and sour are all made from different genes in our DNA. The gene that makes a key bitter taste receptor – called the PTC gene – is found on chromosome 7.1 A change in only three nucleotides in the DNA sequence of this gene makes the difference between tasting and not tasting!
You may now have an excuse for hating your broccoli!
So I just said above that it is a change in only three nucleotides in our DNA that makes broccoli taste differently. What does that actually mean, though? How is someone’s DNA different?
DNA is made up of only four repeating building blocks we call nucleotides. These nucleotides are nicknamed A, T, G, and C (for Adenine, Thiamine, Guanine, and Cytosine).
Nearly every cell that makes up our body contains a complete copy of all of our DNA. A complete DNA copy, also called a genome, is approximately 3 billion nucleotides and 6 feet in length!
People seem so different from each other that you may think their DNA is very different, too. But it’s not. In fact, our DNA is actually 99.9% the same as our friends, classmates, and neighbors! That means there is only a 0.1% difference between you and me. Pretty amazing, huh?
Keeping that in mind, all humans have almost the same exact order of A, G, C, and T through all 3 billion letters. Some of our differences come from the 1 in 1000 nucleotides that are different between people. These differences are genetic polymorphisms.
A polymorphism can be as small as a single change to any one of our 3 billion nucleotides, say, from an A to a T or a G to a C. Or the change can be much larger. In some cases, our DNA is able to handle missing or extra nucleotides ranging in length from 1 to many thousands. These polymorphisms can be either harmless or harmful depending on where they occur.
Here are a few examples of different types of polymorphisms:
Original DNA Sequence
DNA sequence with a change in one letter (T → A)
DNA sequence with a deletion of one letter (T → _)
DNA sequence with an addition of one letter (T → TT)
Most polymorphisms that occur in our DNA are harmless and do not affect our health. Some of these changes cause physical differences in our bodies that we can see, like eye color and hair texture. Other differences are not visible but are important, like our blood type and color vision.
As I hinted, some polymorphisms can be very dangerous to our health. Oftentimes very small changes in the DNA lead to severe diseases.
Let’s quickly look at a couple examples: Cystic Fibrosis and Huntington’s Disease. Cystic Fibrosis is a rare illness in which a thick, sticky mucus builds up in the lungs. A patient with Cystic Fibrosis is usually missing only three nucleotides in their DNA in a region of their chromosome 7.
Huntington’s Disease also results from a polymorphism. When a person has Huntington’s disease, certain cells of the brain, called neurons, die off over time. To get this disease, a string of nucleotides are added into the DNA on their chromosome 4.
You may be surprised that such small changes in our DNA can be so important. But it is these tiny differences that make each of us a unique and interesting person.