I never really understood what happens if you are inbred. I know it means your parents are closely related, but does having closely related parents mess with your DNA?

-A high school student from Michigan

September 21, 2012

That is an interesting question with an equally interesting answer! Having closely related parents doesn’t exactly ‘mess with your DNA’, as you put it. But it does mean that you have less diversity, or variety, in your DNA. And diversity can be very important to your health.

Less variety in your DNA can increase your chances for getting rare genetic diseases. You may have heard of some of these diseases: albinism, cystic fibrosis, hemophilia and so on.

Less variety in your DNA can also make you unhealthy in another way – it can weaken your immune system so you can’t fight off diseases as well.  You can end up a very sickly person!

Now of course inbreeding doesn’t mean you will definitely get a genetic disease or wind up sickly.  You are just more likely to have health problems.  And the more inbreeding, the greater the risk.

So inbreeding doesn’t actually make your DNA change in any way. Instead, inbreeding is risky because it means the DNA from your mom and your dad is similar. And as you’ll see below, when these similar parts come together in their child, this child can end up with problems.

Same DNA, Same Diseases

Every person has 46 chromosomes and each chromosome holds a bunch of genes. Each gene has the directions for one small part of you.  So there is a gene that determines if you’ll have red hair, one that gives color to your skin by making melanin, another one that helps blood to carry oxygen, and so on. 

You actually have two sets of 23 chromosomes.  One set of 23 comes from mom and the other 23 comes from dad. Since each set of chromosomes has the same set of genes*, this means that you have two copies of most every gene. What is important for making us each unique is that the copy you get from your mom can be very different than the copy you get from your dad.

So for example, the gene that causes red hair comes in a red version, and a not-red version (these different versions are called 'alleles').  And the gene that makes a pigment called melanin comes in a normal version that makes melanin and a broken one that doesn't.  If you only have the broken one, you will end up with albinism.

Having two copies of everything is actually a really great system. This is because if one copy is broken, you still have a second copy to use as back up.

This is the case for the gene that makes melanin.  People with just one broken copy don’t have albinism, because their good copy makes enough to keep albinism away. 

But people with one bad gene copy can still pass it down to their kids.  We call these people ‘carriers’, because they carry a single copy, but don’t have the actual disease. And this is where the trouble can start with inbreeding.

If, for example, a woman is a carrier for a broken gene, then she has a 50% chance of passing down this broken version to her child. This doesn’t normally matter so much, because as long as she finds a father with two healthy copies, then their children will always be sure to get at least one healthy copy.

But with inbreeding, it is more likely that your spouse could carry the same broken gene. So in the example of albinism, it would mean that both mom and dad are carriers for the broken gene for making melanin. Then both mom and dad have a 50% chance of passing a broken gene to their child. This translates to each child having a 25% chance for getting the disease (0.5 x 0.5 = 0.25). That’s a pretty high risk!

Now, I’m not saying that people with albinism (or any rare disease for that matter), are always the result of inbreeding. Everyone has five or ten of these broken genes lurking in their DNA. This means it is always a roll of the dice when you pick a spouse as to whether they’ll carry the same broken genes as you do.

But with inbreeding, the risk that you’ll both carry the same bad genes is much higher. Each family is likely to have its own type of disease genes, and inbreeding is an opportunity for two carriers of the same broken gene to pass two copies of it to their children. And then their kid can end up with that disease.

As you can see, it’s good to have babies with someone that has different DNA from you. Then you can give your babies a diverse collection of DNA, and they will always have a back-up allele for any broken ones.

But this isn’t the only reason you want parents to be pretty different genetically.  The second reason you need a lot of variety from each parent is to be able to fight off as many diseases as possible.

Same Genes, Weak Immune System

Having diverse DNA is important for having a strong immune system.  This is why inbreeding can make for some sickly children.  And it is why laboratory mice and some farm animals get sick so easily.

The immune system depends on a very important part of DNA called the 
MHC or Major Histocompatability Complex region.  This is a lot of big words, but basically the MHC region is made up of a bunch of genes that help you fight off disease.

The MHC region’s secret to fighting off disease is to have as many different types of alleles (or versions of genes) as possible. The more variety you have, the better you are at fighting disease.

Diversity is important because each MHC gene is good at fighting a different set of diseases. You can think of it like a lock-and-key system. Each disease is a different shaped lock, and each MHC gene is a key. The more keys you have, the more diseases you can unlock and destroy.

While this may sound very oversimplified, it is quite similar to how our bodies actually work. Our bodies are constantly trying to detect foreign material in the body. Scientists think that each MHC gene allows us to detect a different type of foreign material.

And even more importantly, each allele of an MHC gene can help detect a different type of foreign material. We don’t fully understand yet what types of foreign material each allele can help detect, but we do know that every unique allele helps to detect a different type.

Now I think you can see why inbreeding can cause problems here.  When inbreeding happens and two closely related people have children, these children are likely to have less diversity in their DNA. Which means these inbred children would have fewer types of MHC alleles (or fewer keys).

With fewer types of MHC alleles, they can detect fewer types of foreign material (or locks). They will be more likely to get sick as they can’t successfully fight off as many diseases. The end result is a more sickly person.

As you can see, diversity is the most important thing lost with inbreeding. Whether it’s to ensure that you don’t get two bad alleles and end up with some rare genetic disease, or if it’s to ensure that you get many different MHC alleles, you need diversity to protect yourself.

By Zoe Assaf, Stanford University

Parents with too similar DNA can have sickly children.

Everyone has 5-10 hidden disease genes lurking in their DNA.

You need a wide variety of genes to tackle as many diseases as possible.