If someone is born with a mutation, like violet eyes, can their offspring have violet eyes too?
-A middle school student from California
May 28, 2010
Like many answers in biology, it depends! In this case, it depends on where the mutation is in the body. And this depends on when the mutation happens.
To be seen, an eye color mutation must obviously be in the eyes. But to be passed on, it must also be in the egg or sperm.
Whether it is only in the eyes or both in the eyes and egg/sperm depends on when the mutation happened. If it happened early in development, then odds are it will be in most of the cells of the body. This includes the egg or sperm.
If the mutation happened late in development, then it might only be in the eyes. This means those beautiful eyes will only be around for a single generation.
As you can tell, it is possible for different cells in your body to have slightly different DNA. This is called being a mosaic. And every one of us is a mosaic to some degree.
To answer your question, I'll need to first talk about how mutations happen and why they can affect the way we look. Then you'll see why sometimes a new trait can die with an individual. And why it sometimes gets passed on.
Mutations are Changes in DNA
Our DNA has the instructions for who we are. So it also has the instructions for making us look the way we do.
These instructions are found in stretches of DNA called genes. Each gene has the instructions for one small part of you.
So there is a gene that can turn your hair red. And a bunch that determine how dark your skin will be. And a bunch of genes that all work together to give you your eye color. And so on.
It makes sense then that changes in these genes can sometimes change what we look like. A change like this is called a mutation.
Mutations can and do happen in many different ways. Sometimes things like sunlight or cigarette smoke damage our DNA. And sometimes our cells make a mistake and cause a mutation all by themselves.
When cells make new copies of themselves, they need to copy their DNA too. The cell copying machinery is very accurate but sometimes it makes a mistake.
Usually the cell's repair machinery can fix a mistake before it becomes a problem but not always. A few inevitably slip through and become part of the cell's DNA.
These kinds of mistakes can happen anytime. They can happen in one of your skin cells right now. Or they might have happened when you were just a fertilized egg.
When a mutation happens is going to affect which cells have it. If your skin cell picked up a mutation right now, only cells that came from that skin cell would have it. But if the DNA in your egg or sperm got changed, then all of your kids would get that change.
From One to Many
Let's travel back to when you were inside your mom's womb. When your dad's sperm fertilized your mom's egg, this made the first cell of what later became you. This means you started from one single cell.
This first cell was called the zygote. You, as the zygote, then divided over and over during development to create your whole body.
So one cell became two, then these two cells split into four, and so on until you get to the trillions of cells that make up you. Every cell in your body today came from that first cell through a series of cell divisions.
Now let's think about how the DNA travels through the cell divisions. The first cell that became you had one set of DNA instructions. So you started out with a single set of DNA.
When that first cell divided into two, the DNA was copied and one copy went into each cell. This happened trillions of times until every cell in your body got a copy of the DNA. That means all the DNA in your body came from the DNA of the zygote.
The DNA in the zygote that became your entire body came from your parents' DNA inside their sperm and egg. Likewise, the only part of your DNA that is passed on to the next generation is in your sperm or egg. This means the only mutations that matter for your offspring are mutations in the cells leading to sperm or egg (or the sperm and egg themselves).
As a zygote develops into an embryo, cells that make different parts of the body split off from one another. When the embryo is at 32 cells, each cell already knows whether it will eventually be the tissue that will make a brain cell or lung cell, for instance.
The cells leading to sperm and egg split off pretty early during development. So for the mutation for violet eyes to be passed on to offspring, there are two different scenarios. 1) The mutation would have to happen before the sperm or egg split off during development. 2) It would only happen in the cell lineage leading to sperm or egg.
In the first case, both parents and children would have violet eyes. In the second, only the children would have the trait. It would seem that violet eyes just suddenly appeared. Which they did. If the mutation happens later in development or in cells besides sperm or egg, it will not be passed on. For instance, if the violet eye mutation only happens in the DNA in your eyes, this mutation will die with you.
Just because a mutation isn't passed on, that doesn't mean it isn't important to the person who has it. Sometimes mutations can do good things. Like the person turns those violet eyes into a movie career. But sometimes mutations can do something bad like cancer.
Cancer, which is one of the leading causes of death in the world, happens because of mutations in your DNA in cells other than sperm or egg. This is why sunlight can cause skin cancer or smoking cigarettes can cause lung cancer.
Luckily though, cancer can't be passed on for the same reason that a mutation that is only in your violet eyes can't. The mutation that causes cancer is almost never in the sperm and egg. Which means the cancer dies with the person who has it.
Now that isn't to say that cancer can't run in families. It can. But what runs in families is an increased chance that a mutation will cause cancer, not the cancer itself. So if a parent gets cancer, it usually does not mean a child will for sure end up with cancer too.
As you can see, mutations in our DNA are really important. Mutations can be good, like beautiful violet eyes. Or bad like cancer. Either way, they affect how we look and help determine our quality of life.
Dan Kvitek, Stanford University