Genes and Hearing Loss

Genes are highly complex and form a roadmap of sorts for the synthesis of proteins, which are the building blocks for everything in the body: hair, eyes, ears, heart, lungs, etc. Every child inherits half of their genes from one parent and half from the other parent. If the inherited genes are abnormal or defective, a health disorder such as hearing loss or deafness can occur. Hearing disorders are commonly inherited in one of four ways:

1. Autosomal Dominant Inheritance—For autosomal dominant disorders, only one “bad” copy of the gene is needed to affect the child. A “heterozygous” parent has two types of the same gene (in this case, one mutated and the other normal), each with a 50 percent chance of affecting the child. An affected homozygous parent (who has two same affected genes) will pass an abnormal copy to all their children, and they will all be affected. Autosomal dominant traits usually affect males and females equally.
2. Autosomal Recessive Inheritance—For autosomal recessive disorders, the child must inherit a bad copy of the gene from both parents. If there is one good copy of the gene, the child will not be affected. It’s possible for two unaffected heterozygous parents (each parent with one bad copy) to have an affected child (a 25 percent chance), which means that this disorder can skip generations.
3. X-linked Inheritance—These disorders are from genes that are on the sex chromosomes. Males have an X chromosome and a Y chromosome, while females have two copies of the X chromosome only. Each female inherits an X chromosome from her mother and her father. On the other hand, each male inherits an X chromosome from his mother and a Y chromosome from his father. Genes that are only on the X chromosome are more likely to affect males because they don’t have a competing gene on another chromosome.
4. Mitochondrial Inheritance—Mitochondria also have their own DNA. Only the mitochondria in the egg from the mother can be passed from one generation to the next. This leads to a pattern where only affected mothers (not affected fathers) can pass on a disorder from one generation to the next. Being sensitive to hearing loss from aminoglycoside antibiotics, which are used to treat serious infections, can be inherited through mitochondrial DNA. This is the most common cause of deafness in China.

Advances in molecular biology and genetics over the past 10 years have helped scientists and doctors understand how the inner ear develops, works, and how it can be abnormal. Researchers have identified several genes responsible for deafness or hearing loss, including the “GJB2” gene mutation. As one of the most common genetic causes of hearing loss, GJB2-related hearing loss is an autosomal recessive genetic disorder because the mutations only cause deafness in individuals who inherit two copies of the mutated gene, one from each parent. A person with one mutated copy and one normal copy is a carrier but is not deaf. Screening tests for the GJB2 gene are available for at-risk people to help them determine their risk of having a child with hearing problems. Additionally, panels to screen for genetic hearing loss have been developed if you or your healthcare provider suspect that there may be a genetic cause for the hearing loss.