Autosomal dominant

Contents

What is autosomal dominant

What is autosomal dominant

Autosomal dominant means one copy of the abnormal gene from only one parent or in each cell is sufficient to cause the disorder or disease. In some cases, an affected person inherits the autosomal dominant condition from an affected parent. In others, the autosomal dominant condition may result from a new mutation in the gene and occur in people with no history of the disorder in their family. This is called a de novo mutation. Huntington’s disease, Marfan syndrome and neurofibromatosis type 1 are common examples of an autosomal dominant genetic disorders.

Autosomal refers to the fact that whatever gene is involved is found on one of the first 22 chromosomes (called the autosomes) and not on the X or Y chromosome (the sex chromosomes).

Dominant refers to the above explanation that you have two copies of each gene, one from mom and one from dad, and in order to have an autosomal dominant condition, a person only has to have one copy of the abnormal gene. They can inherit this copy from mom or dad, who may also have the condition.

What does autosomal dominant mean?

Often autosomal dominant conditions can be seen in multiple generations within the family. If one looks back through their family history they notice their mother, grandfather, aunt/uncle, etc., all had the same condition. In cases where the autosomal dominant condition does run in the family, the chance for an affected person to have a child with the same condition is 50% regardless of whether it is a boy or a girl. These possible outcomes occur randomly. The chance remains the same in every pregnancy and is the same for boys and girls.

When one parent has the abnormal gene, they will pass on either their normal gene or their abnormal gene to their child. Each of their children therefore has a 50% (1 in 2) chance of inheriting the changed gene and being affected by the condition.
There is also a 50% (1 in 2) chance that a child will inherit the normal copy of the gene. If this happens the child will not be affected by the disorder and cannot pass it on to any of his or her children.

There are cases of autosomal dominant gene changes, or mutations, where no one in the family has it before and it appears to be a new thing in the family. This is called a de novo mutation. For the individual with the condition, the chance of their children inheriting it will be 50%. However, other family members are generally not likely to be at increased risk.

Figure 1 illustrates autosomal dominant inheritance. The example below shows what happens when dad has the condition, but the chances of having a child with the condition would be the same if mom had the condition.

Key points to remember

A person only needs to inherit one copy of the abnormal gene in order to be affected by the condition (50% chance). These outcomes occur randomly. They remain the same in every pregnancy and are the same for boys and girls.
A abnormal gene cannot be corrected if it is present for life.
A abnormal gene is not something that can be caught from other people. They can still be a blood donor, for example.
People often feel guilty about a genetic condition which runs in the family. It is important to remember that it is no-one’s fault and no-one has done anything to cause it to happen.

Figure 1. Autosomal dominant inheritance

Why does a genetic condition sometimes appear to miss out a generation?

Some dominant genetic conditions can affect family members very differently. This is called variable expression. The condition does not actually miss out a generation, but some people have such mild symptoms of the condition that they appear to be unaffected. They may not even know that they have the condition.

In conditions which occur later in life (adult onset conditions e.g. inherited breast cancer and Huntington’s disease), people may have died earlier of unrelated causes leaving no time for the condition to appear, or the correct diagnosis may never have been given. However, the parents may have passed on the condition to their children.

What if a child is the first person in the family to have the condition?

Sometimes a child born with a dominant genetic condition can be the first person to be affected in the family. This may happen because a new gene change has occurred, for the first time, in either the egg or the sperm that went to make that child. When this happens, the parent of that child is not affected. The parents are very unlikely to have another child affected by the same condition, but you should always discuss the risks with your doctor. However, an affected child, who now has the changed gene, can pass it on to his or her children.