-A high school student from California
March 17, 2010
This is a question that a lot of people wonder about. I wish I could give you one number to answer your question. But the chance of a child being born with a genetic disorder depends on a number of things.
These things include what DNA each parent passes down to their child as well as the condition of that DNA. Random chance plays a big role too.
The chances a child will end up with a genetic disorder is a combination of all of these things. So each
child of each
set of parents faces a different risk.
A number bandied about is that there is a 3-5% chance a child will be born with a genetic disorder. In fact, this number is the chance a child will be born with a birth defect. This birth defect may or may not be genetic.
This is also an average of everybody's risk. But each couple is going to have a different risk each time they have a child. This depends on all of the factors I talked about before.
It is very difficult to figure out the chance a child will have a genetic disorder. But it does get a little easier if a disease is known to run in a family.
What I'll do for the rest of the answer is talk about each of the factors that contributes to your risk. As you'll see, we have control over some but not all of them. And we can really only figure out a good number for a few. For now anyway.
The Genes We Inherit
For the risk of some genetic conditions we can look to our mom and dad. Their genes " and their health " can tell us a lot. But their genes can be hiding things too.
If someone in the family has a genetic condition, then we do many different things
to try to figure out the chances that your child might get it too. In the end we may even be able to give a fairly accurate number about the child's risks for that condition. Something like 25% or 50% for example.
If no one in the family has a genetic condition, then the chances that a child will have it are probably very, very small. The chances aren't zero though because some genetic conditions can be hidden within a family.
For a child to be affected with one of these hidden conditions, both mom and dad have to carry a genetic difference. We don't always know if mom and dad have these differences. But we can still calculate a general risk.
Often we do this based on ethnicity. For example, cystic fibrosis
(CF) is more common among Caucasians. If a child has two Caucasian parents, the chance they will have CF is about 1 in 3000. If a child has two African American parents, then the chances are much less -- about 1 in 15,000.
Again this risk is a general risk and may not apply to your exact situation. For example, if both parents do have a hidden version of a gene that can lead to CF, then each of their kids has a 1 in 4 chance for getting CF. If one or neither parents carries this hidden version, then each child has an extremely low chance of getting the disease. So everyone's specific risk is different.
Still, the odds based on ethnicity can be useful. They can help us decide whether to get tested for certain genetic disorders before getting pregnant. Or to help us decide if we may want to test a child before it is born.
The Condition of the Genes We Pass On
Sometimes there are other things that influence the chance for a child to have a genetic condition. These are things like the age and health of a parent. Both can affect the DNA that is passed on to a child.
As women get older, the chance that they will have a child with an extra chromosome goes up. The effect of this extra chromosome can vary.
Sometimes in cases like Klinefelter's or XXY, there is very little effect. Other times it can range from disability to death. It all depends on which genes are on the extra chromosome.
When men get older, changes can take place in the DNA they pass on too. These tend to be smaller changes than those in women but can definitely cause problems too. For example, one study
suggested that most cases of achondroplasia (the most common kind of dwarfism) happen because of a change in a father's DNA.
can also affect genes. Sometimes the things we do or that happen to us can damage our DNA.
For example, we do know that too much sunshine can cause a change in our genes. Smoking can do the same thing. If the damage is in the wrong place, it can cause a genetic disorder. And if that DNA damage happens in the egg or sperm, then it can be passed on to the next generation.
The environment can also affect our DNA in ways that scientists don't totally understand yet. These changes can affect how our DNA is used and can be passed on to our kids too.
To some extent we can control these things but not totally. For example, we can minimize some risks by having kids earlier in life. But we can't control where we grew up as a kid, what our mothers did while they were pregnant with us, etc.
Sometimes changes happen in our genes just by accident. As the cells in our body grow, they make new copies
of our genes. Mistakes can happen in this process. Nothing we do or don't do causes the change.
In general, these kinds of spontaneous changes don't happen that often. And they are passed down even less often.
Scientists have calculated
that the chance of a DNA change being in an egg or sperm DNA is about 1 in 20. So the chances that a fertilized egg will have a DNA change is 1 in 10.
That may seem like a lot but it really isn't. We have over 6 billion letters of DNA in each cell and genes are only a small part of that DNA. This means the odds of hitting a gene aren't that high.
We also need to remember that most of our 20,000 or so genes have a back up copy. So even if we have a change in our genes, it is most likely we won't even know it!
Calculating the Risk
We each have a unique genetic code. And we each have a unique risk for developing a genetic condition. That risk is different even for brothers and sisters with the same parents. Why? Because the genes we get from our mom and dad are only part of the puzzle.
To answer your question, we need to sum up all of the things I've talked about: the genes we inherit, the condition of the genes we inherit and random chance. We can calculate risk numbers for some of them. For others, we have to guess. It's not easy!
Scientists say in the next few years we will all know our unique genetic code. This will certainly tell us more about the DNA we inherit. It may tell us a little more about the DNA we can pass on. But there's still random chance. Even our best calculations can't control that.