Genes are the blueprints for making the substances, called proteins, our bodies need to develop and work properly. Most genes come in pairs, one of which is inherited from the mother and the other from the father. A mutation is a change in a gene that causes it to be different from the copy inherited from either parent. Genes that are inherited from our biological parents are expressed in specific ways. One of the basic patterns of gene expression is called autosomal dominant inheritance.
There are two types of chromosomes: the sex chromosomes, which determine male or female gender, and all the other chromosomes (chromosome pairs 1 through 22). Autosomal inheritance of a gene means that the gene is located on one of the autosomes. This means that males and females are equally likely to inherit the gene. "Dominant" means that having one of the two copies of a particular gene is all it takes for a person to have a trait, such as brown eyes instead of blue eyes. When a parent has a dominant gene, there is at least a 50 percent chance that any child he/she has will also express the trait.
There are four possible combinations in the children. Two of the four, or 50 percent, have inherited the gene. The other 50 percent have not inherited the gene. 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 autosomal genes.
A characteristic of some dominant genes is that they can have variable expression. This means that some people have milder or more intense characteristics than others. Another important characteristic of dominant genes is that in some cases, they can have reduced penetrance. This means that sometimes a person can have a dominant gene copy but not show any signs of gene. 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 he or she will automatically develop cancer. It simply means that the person has inherited a mutation in a gene that gives him or her a higher chance to develop cancer than someone without the mutation.