CMT is a genetic disorder, meaning it’s caused by a change, or mutation, in your genes. If you have CMT, you might be wondering how you got it. It’s likely that other people in your family have it, too — though this isn’t always the case. To understand how and why, you must know a bit about genetics.
Genes: What Make You “You”
Each person has a unique code that makes us who we are. This code is written in our genes. Our genes determine what we look like, how our bodies function and whether we have (or are likely to develop) certain diseases.
Each gene has a special role. Some genes are responsible for things like forming bones, making blood and helping you breathe. Other genes are responsible for physical traits, like hair color, eye color and height.
The vast majority of genes are the same in all humans. Less than 1% of our genes are different, and it’s these genes that make us each so unique.
Genes and Heredity
Almost every cell in your body has two copies of every gene: one from your mother and one from your father.
Remember that most of our copies — more than 99% of them — are the same in every human, so most of these copies are the same, too. But sometimes the copy we get from our mother is different from the copy we get from our father.
So what happens when we have two different copies of a gene? Let’s take the gene for eye color as an example. (In reality, there are many genes that are responsible for eye color, but for the sake of simplicity, we’ll pretend there’s just one.) Say your mom has blue eyes and your dad has brown eyes. You inherit one gene for blue eyes from your mom, and one gene for brown eyes from your dad. What color will your eyes be?
Most likely, your eyes will be brown. This is because brown eye color is a dominant form of the gene, while blue eye color is a recessive form. You need only one copy of a dominant gene for what it codes for — in this case, eye color — to be expressed. In most cases, you would need two copies of the recessive gene — a copy of the blue eye gene from both mom and dad — for that gene to be expressed.
Genetic Mutations
Some people will have a change in a gene that makes it different from everyone else’s. This change is called a mutation. Some genetic mutations are hereditary, meaning they can be passed down from a parent to their child.
CMT is caused by hereditary genetic mutations. But just because someone has CMT doesn’t mean they got it from one of their parents. Sometimes the mutation can happen randomly before they are born. This is called a spontaneous mutation. Someone with a spontaneous mutation can still pass it on to their children.
How is CMT Passed Down?
There are many different types of CMT and they are passed down, or inherited, in different ways.
The way that CMT is inherited is called its inheritance pattern. Below are the three distinct inheritance patterns of CMT.
Autosomal Dominant Inheritance: More Common
Most cases of CMT are autosomal dominant. In autosomal dominance, the mutation occurs in a dominant form of the gene, so only one copy of the gene with the mutation is needed to cause CMT.
In these cases, one parent usually has CMT and the other doesn’t. The parent with CMT has one gene with the mutation and one without the mutation. Each child they have will have a 50% chance of having CMT. If their child inherits the gene with the mutation, they will have CMT. If they inherit the one without the mutation, they will not have CMT, and they will have no chance of passing CMT on to their children.
Autosomal Recessive Inheritance: Less Common
CMT4, some subtypes of CMT2 and some other CMT cases are autosomal recessive. This inheritance pattern is rare. These mutations occur in a recessive form of the gene, so someone with one of these types of CMT must have two copies of that gene. Usually, this happens when both parents have one copy of the gene with the mutation and they each pass that gene on to their child. In these cases, the parents are called carriers — they don’t have CMT, but they carry the recessive gene that causes it.
Someone who is a carrier can only have a child with CMT if the other parent has CMT or is a carrier. If both parents are carriers, their child has a 25% chance of having CMT. If only one parent is a carrier, their child will not have CMT, but the child will also be a carrier 50% of the time.
X-Linked Dominant Inheritance
CMTX is inherited in an X-linked dominant pattern. In these cases, the CMT-causing mutation is located on the X chromosome.
All humans have two sex chromosomes. Women have two X chromosomes and men have an X and a Y chromosome. If a woman has X-linked CMT, she can either pass on the normal X chromosome or the X chromosome with the mutation, so there is a 50% chance her child will have CMT, regardless of the child’s sex. But if a man has X-linked CMT, all of his daughters will have CMT since they inherit his X chromosome, and none of his sons will have it since they inherit his Y chromosome.
Genetic Testing for CMT
If you have CMT, you might want to consider genetic testing. In many cases, genetic testing can identify the type of CMT you have, which will help you understand more about your specific condition and how it’s likely to progress. It can also help you make important family planning decisions.
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