Autosomal dominant inheritance means that the gene carrying a mutation is located on one of the autosomes (chromosome pairs 1 through 22). This means that males and females are equally likely to inherit the mutation. "Dominant" means that having a mutation in just one of the two copies of a particular gene is all it takes for a person to have a trait, such as an increased risk of developing cancer (see explanation below on "variable expressivity" and "reduced penetrance"). When a parent has a dominant gene mutation, there is a 50 percent chance that any child he/she has will also inherit the mutation. There are four possible combinations in the children. Two of the four, or 50 percent, have inherited the mutation. The other 50 percent have not inherited the mutation. 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. An important characteristic of dominant gene mutations is that they can have variable expression. This means that some people have milder or more severe symptoms than others. In addition, which systems of the body the mutation affects can vary as can the age at which the disease starts, even in the same family. Another important characteristic of dominant gene mutations is that in some cases, they can have reduced penetrance. This means that sometimes a person can have a dominant mutation but not show any signs of disease. The concept of reduced penetrance is particularly important in the case of autosomal dominant cancer susceptibility genes. If a person has inherited a cancer susceptibility gene, it does not mean they will automatically develop cancer. It simply means that they have inherited a mutation in a gene that gives them a higher chance to develop cancer than the general population (i.e., someone without the mutation). 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. When a cancer susceptibility gene mutation is inherited in an autosomal dominant manner, it means that the mutation can be inherited from the mother, or the father, who themselves may or may not have ever had any type of cancer. However, with autosomal dominant inheritance, if a parent does not have the gene mutation associated with cancer risk in the family, he/she cannot pass it on his/her children.