Your DNA contains two copies of every gene - one inherited from each parent. These genes sometimes pass on inherited conditions, caused by changes in genes called mutations. In most cases, as long as the carrier carries only one copy of a gene that contains a mutation, symptoms will not be present. While carriers are usually healthy, they can still pass down condition-causing gene mutations on to their children even though neither parents have any symptoms. This is because everyone carry mutations in their DNA. Some of these mutations however, can impact the health of the children who inherit them. This usually happens if both parents have a mutation in one copy of the same gene. In such cases, there is a 1 in 4 chance for each pregnancy that a child will be affected by the condition associated with the gene.