Genes come in pairs. If one gene of a pair causes, by itself, a particular trait or disorder to be present, it is called ‘dominant.’ If the gene is found on one of the first 22 pairs of chromosomes, it is called autosomal dominant (chromosomes 1 through 22 are called ‘autosomes’). Although genes are always in pairs, in autosomal dominant inheritance, you only need to inherit one copy of a dominant gene in order to show a particular trait.
Autosomal dominant conditions can be inherited from one parent, who also has that condition or the gene for that condition, or can occur in one person in the family for the first time (called a ‘new mutation’). If neither parent has the autosomal dominant gene change that the child has, it is a ‘new mutation’ just present in that child, and there is a low risk for it to affect other siblings. However, the person with the autosomal dominant gene change has a 50% chance to pass it on to each of his or her children.