If people get some genes from their mom and some genes from their dad, do you have some of your own genes?
-A middle school student from CaliforniaSeptember 2, 2009
Mostly you have the same genes as either your mom or dad. But you do indeed have a few that are all you.
Now I don't mean you have any genes never before seen in people. There are ways this can happen but this is usually a slow process that wouldn't happen in one generation.
Instead I mean that you have versions of a few genes that neither your mom nor your dad has. See, all people have pretty much the same genes. What makes you different from me is that we each have different versions (or alleles) of these same genes.
To see why a few of your genes are unique, we're going to have to step back and go over what genes are and how they are passed down. Then we can see the many ways that new gene versions can happen.
Same Genes, Different People
A gene is a stretch of DNA that has the instructions for making a particular protein. Each protein has a specific job it does.
For example, one gene has the instructions for making a protein that carries oxygen in our blood. Another gene has instructions for a protein that makes a dye that gives our eyes color. And so on for most everything your body does.
As I said before, all humans pretty much share the exact same genes. What makes each of us different is that we have different versions of these same genes.
Let's look at eye color as an example. There is an eye color gene called HERC2. It comes in two different versions, brown and blue. So brown eyed people have brown versions of HERC2 and blue eyed people have blue versions. Same gene, different versions.
And having different versions of the same genes isn't the only way we are each unique. Another has to do with the fact that we have two copies of each of our genes.
Judging by your question, I am sure you have heard that you get half your genes from your mom and half from your dad. This doesn't mean you get different genes from each parent. Instead you get two complete sets of genes -- one from mom and one from dad.
This is important because some gene versions are more powerful than other ones. They are dominant.
To see why this matters, let's look at the eye color gene again. We each have two copies of the HERC2 gene, one from mom and one from dad. If we get a brown from each parent, then we have brown eyes and if we get a blue from both, then we have blue eyes.
If we get one of each, then we have brown eyes. Because the brown version of HERC2 is dominant over the blue one. It is because of different versions and having two copies of each gene that brown eyed parents can have a blue eyed child.
But a blue-eyed child with brown-eyed parents doesn't usually have a new version of the eye color gene. This child still just has the same versions that his or her parents had but in a new combination that gives blue eyes instead of brown. To really get at how you can end up with a gene version that neither of your parents has, we need to learn a bit about how we get our genes from our parents.
Genes are Passed Down in Chromosomes
Genes are found on long strands of DNA called chromosomes. Most humans have 23 pairs of chromosomes. So they have two copies of chromosome 1, two copies of chromosome 2, etc.
You get one copy of each of your chromosomes from your mother and one from your father. This is how you get two copies of most every gene.
Recombination means that everyone has unique chromosomes.
Chromosomes are passed onto children in a complicated process called meiosis. The cells that lead to an egg or a sperm have to first copy all of their chromosomes. Then, before one of each chromosome gets randomly put into the sperm or egg, something called recombination happens.
Recombination is when sections of DNA can be randomly 'cut-and-pasted' between the chromosomes in each pair. So the chromosome 1 we get from mom is a unique mix of her two chromosome 1's. And this is true for all of your chromosomes you got from mom and dad. Each of your chromosomes has never been seen before.
Copying and recombination can and do sometimes create new gene versions. Let's see how.
DNA Swapping can Create New Gene Versions
Sometimes recombination can happen in the middle of a gene making something new. Or it can change how a gene works.
Let's take the case of the eye color gene again. If both your parents have blue eyes, you will almost always get one blue version from each of them. In that case you would also have blue eyes. But thanks to recombination, you could actually end up with brown eyes.
Recombination can give rise to new gene versions.
Imagine that one of your parents has two blue eye versions of the eye color gene. One of these has a DNA difference at the front end of the gene that causes blue eyes and the other has one of these differences at the back end. Each copy has a single difference but at different places in the gene.
Now imagine that when the sperm or egg is being made, the front half of one gene gets pasted onto the back half of the other. This would result in one brown version without any differences, and one blue version with two differences (which could still give blue eyes). This means that two blue-eyed parents have now produced a brown-eyed child. And that the child now has a version of an eye color gene that neither parent has.
So you can see how recombination can result in you getting different alleles than your parents. Depending on the versions being swapped, this can change how the gene works.
I have mentioned eye color as one example. Click here to read about how this can happen with blood type.
DNA Mutations are Common
Besides recombination, another way to get your own gene versions is through mutations. A mutation is simply a change in the DNA. And they are surprisingly common.
DNA is made up of 4 building blocks, the chemical bases A, T, G and C. Mutations can sometimes change just one of these -- maybe a G to an A, for example. Or a base can be added or taken away. Sometimes big chunks of DNA can be added or go missing.
There are many ways mutations can happen. One way is because of damage from the environment. This is why sunlight can cause skin cancer, for example.
Another way mutations can appear is when DNA is being copied. Our DNA copying machinery is very good, but every once in a while a mistake can creep in. If the mistake isn't fixed, your DNA ends up with a mutation.
DNA mutations sound scary but they usually aren't. Some can be harmful but most have no effect. And a few can even be useful. (Click here for more detail about mutations.)
Also, mutations don't have to happen in a gene although they sometimes do. If the DNA is mutated in one of your genes, odds are you will now have a version of that gene different from your mom and dad's.
Mutations can give rise to new gene versions.
Let's look at eye color again and our blue eyed parent. Remember, this parent has a blue version of HERC2 that has a single DNA difference. Imagine that while this parent's DNA was being copied and put into an egg or sperm, the cell made a mistake at the exact spot where the difference was.
Now the blue version has been converted to brown in this particular egg or sperm. If it goes on to be part of a new child, then this child will have an eye color gene different from either parent.
Now I should say that a new gene version doesn't necessarily have to have an effect. For example, someone may get a mutation in a gene that has no effect. This is not an exciting new version but it still counts as something your parents didn't have.
So you do have a few unique genes because of recombination and mutations. And it is through processes like these that all of the wonderful variety around us came from.
In fact, these processes can even explain one way that new genes can form. Sometimes recombination or mutation can cause bigger chunks of DNA to get duplicated or to go missing. Sometimes the DNA is so big it includes a whole gene.
A duplicated gene is free to get mutations without worrying too much about what happens to the person. If a mutation causes the gene to stop working, there is the original still there to make up for it.
As mutations build up in a duplicated gene, it can become so different that it is a new gene. Perhaps you have a few started right now in you. And so does everyone else around you.
Sandeep RavindranFun video about the DNA differences (SNPs) between people
This project was supported by the Department of Genetics, Stanford School of Medicine. Its content is solely the responsibility of the authors and does not necessarily represent the official views of Stanford University or the Department of Genetics.