Many parents are unfamiliar with how hearing loss could be caused by genetic factors. As you work to identify the cause of your child's hearing loss, it will be helpful to have a basic understanding of genetics and the major categories of inheritance.
Roughly 50% of childhood hearing loss is genetic. There are over 400 known genetic causes involving hearing loss. The number of genes known to cause hearing loss is constantly changing as researchers identify them.
Genetic scientists categorize hearing loss into two general types: Non-Syndromic and Syndromic. By far, the more common is non-syndromic hearing loss which includes 2/3 of all genetic hearing losses.
Autosomal recessive hearing loss can occur when a child receives a hearing loss gene from each parent. Sibling children born to the same couple have a 25% chance to have hearing loss. Typically the parents have normal hearing and are called unaffected carriers. It can be difficult to understand that a condition can be "Genetic" yet not previously seen previously in the family.
Males and females have the same 50% chance of having hearing loss. Autosomal dominant hearing loss can run through families from generation to generation or it can occur in a family for the first time as the result of a new gene change. This type of genetic hearing loss can vary greatly among affected family members.
X-Linked (or Sex-Linked) hearing loss occurs when the altered gene lies on the X chromosome. Females have two X chromosomes and rarely show the effects of an altered gene on the X chromosome. Males have only one X chromosome and will show the effects of an altered gene on the X chromosome.
In addition to genes lying on chromosomes, every person has genes in the mitochondria. Mitochondria are structures in the cell that produce the energy that cells need to survive. Errors in the mitochondrial genes usually affect functions that require a great deal of energy including the ears, eyes, kidneys, muscles and heart. These changes in mitochondrial genes can result in syndromes involving hearing loss.
Mitochondrial genes are passed to the next generation in egg cells. An egg cell may have few to many altered mitochondrial genes. The specific mitochondrial gene alteration and how many altered mitochondrial genes determine the severity of the condition.