Inheritance Patterns and CMT

The most common forms of CMT are inherited in an autosomal dominant pattern. Autosomal means that the mutation occurs on a chromosome other than the X or Y chromosome. An affected person has one normal gene and one CMT gene in the relevant pair and each child has a 50/50 chance of inheriting the abnormal or CMT gene, giving them the disorder. Either boys or girls have an equal chance of inheriting the disease in this inheritance pattern. In this form, affected children can pass the CMT gene to their children, but unaffected children do not have the abnormal gene in their DNA and cannot pass CMT to their children.

The least common forms of CMT are inherited in an autosomal recessive manner. All Type 4 patients have inherited the autosomal recessive form of CMT. In autosomal recessive forms, both parents have to be “carriers” of the defective gene before a child can be affected. Neither parent shows signs of any symptoms of CMT, but the child who inherits autosomal recessive CMT gets a double dose of the defective gene causing him or her to have the disease, usually in a more severe form. Two parents each with an autosomal recessive CMT gene have a 1 in 4 chance of passing it on to their children. It can affect both males and females.

The final form of inheritance is called X-linked. In this form, the faulty CMT gene is carried on the female sex chromosome. The disease is inherited by males from their mothers, who are carriers. The mother will typically have no symptoms at all or be mildly affected. Each of her sons will have a 50 percent chance of having CMT, and an affected son is likely to be more severely affected than his mother. Each of the girls has a 50 percent chance of being a carrier.

A male affected with X-linked CMT cannot pass the disorder to his sons, but all of his daughters will be carriers of the disorder. Carrier females have a 50 percent chance of passing the disorder to their sons. Unaffected children cannot pass the disorder onto their children since they don’t carry the gene.