What causes autosomal recessive genetic disorders?

Most genes in our bodies come in two copies, one copy which we inherited from our mother and one copy which we inherited from our father. For these types of disorders, an affected person has to have a mutation in it in both copies of the gene in order to be affected.  Parents with a child affected by an autosomal recessive genetic disorder typically do not have the disorder themselves. Instead, they are carriers. To be a carrier means that the parent has a mutation in only one of their two copies of the gene.  The one unmutated copy of the gene makes enough of a needed protein so that they don't develop symptoms; it can be thought of as a back-up copy or a spare.  Since carriers never develop symptoms, they are usually unaware that they have a mutation and are at risk to have an affected child.