Parents are sometimes very confused as to why they would have a child with hereditary deafness when there are no deaf people in either family. This brings us to the issue of recessive inheritance and de novo mutations in hereditary deafness. The majority of non-syndromic deafness is autosomal recessive. If each parent carries one of the genes that cause deafness, they themselves are only carriers and do not develop the disease, but there is a 25% chance that both genes will be passed on to the child and the child will develop hereditary deafness. Another rare case is when an embryo is infected by a virus, environmental toxicity factors, etc. during development causing a de novo mutation in the gene, which leads to deafness in the child.