Autosomal dominant: myotonic dystrophy

Genes are inherited from our biological parents in specific ways. One of the basic patterns of inheritance of our genes is called autosomal dominant inheritance.

What is autosomal dominant inheritance?

Autosomal dominant inheritance means that the gene is located on one of the autosomes (chromosome pairs 1 through 22). This means that males and females are equally affected, and "dominant" means that only one gene is necessary to have the trait. When a parent has a dominant trait, there is a 50 percent chance that any child they have will also inherit the trait:

There are four possible combinations in the children. Two of the four, or 50 percent, have inherited the trait. The other 50 percent have not inherited the trait. These four combinations are possible every time a pregnancy occurs between these two individuals. The gender of the children (whether they are sons or daughters) does not matter. The chance is 50/50 for each pregnancy.

What is myotonic dystrophy?

An example of an autosomal dominant condition is myotonic dystrophy. Myotonic dystrophy is an inherited disorder that causes muscle weakness and myotonia (inability of muscles to relax after use), which becomes more severe over time. Specific problems in other systems of the body can also occur. Myotonic dystrophy is known as an extremely variable condition. This means the severity varies (some people are mildly affected; others are very severely affected), which systems of the body it affects can vary, and at what age the disease starts can vary, even in the same family. This is a characteristic of dominant traits: they can have "variable expression". Most families know that there is a dominant trait or disorder in their family, because it is passed from parent to child and can be seen in many generations. It is important to remember that with autosomal dominant inheritance, if a person does not have the trait, they cannot pass it on.