How genes can cause deafness
Genes can be described as instructions which tell our body how to grow and work. Many of our genes are involved in the development of the ear and in hearing. If one of these genes is different from the usual code, it can cause deafness. More than half of deaf children born in the UK are deaf because of a genetic reason.
Deafness can be inherited, meaning it’s passed down in families, even where there appears to be no family history of deafness. However, gene variants which cause deafness can also arise for the first time in a child, meaning there’s a genetic reason, but it’s not inherited from a parent.
Genetics is a complicated field that’s always changing. Your doctor, audiologist or genetic counsellor can give you more specific information relating to your child and your family or can refer you on to someone else who can.
Our bodies are made up of cells, which are like tiny building blocks that give our body structure and convert nutrients from food into energy. Almost all our cells contain a complete set of our genetic material.
Genes are made of the chemical called DNA and act as instructions which tell our body how to grow and function. We each have around 20,000 genes which are packaged up into chromosomes. There are 23 pairs of chromosomes, and we inherit one of each pair from our mother and one from our father. This means we have two copies of most of our genes, one inherited from each parent (the exception is genes carried on the X chromosome – see X-linked inheritance below for more information about this).
To give you a visual picture, think of each cell in your body as having a library of information at its centre. The chromosomes are like the shelves, each containing thousands of books. Each gene is like a separate recipe book which gives information on how to make one specific protein or substance needed by the cell. The DNA is like the paper and ink that make up the books. Not all the DNA (‘paper’) on the shelves is useful – a lot of it is not coding information – so there are plenty of gaps between the useful genes or ‘books’.
Most genes function the same in everyone, for example those that control how we break down food and generate energy. A smaller proportion of genes produce variations in the proteins they code for. These variations contribute to each person’s unique physical features such as eye colour or height.
A gene variant is a permanent change in the DNA sequence that makes up a gene. We all carry some variation in our genetic material, and most of these gene variants have no impact on our health or development. However, some can cause genetic conditions or genetic diseases such as cystic fibrosis or sickle cell disease. A gene variant that causes a recognised condition may also be referred to as a gene alteration or a gene mutation (the latter is the scientific term).
Gene variants may be inherited, but this is not always the case. For example, a gene can acquire a change within it if an error is made as the DNA copies itself during cell division.
We have around 20,000 genes. Of these it’s estimated that at least 200 are involved in the ear’s development or function. A variation in either one or both of a pair of genes (depending on the role of the protein that gene pair codes for) can cause deafness. The different ways deafness can be inherited are explained further below.
The most common way deafness is passed down in families is when two parents each pass on a recessive gene. This is known as autosomal recessive inheritance (‘autosomal’ means that the gene is located on a pair of the non-sex chromosomes).
Many genes work in a recessive way. This means that, as long as someone has at least one working copy of that specific gene pair, the cell produces enough working protein so the variant (‘non-working’) gene is effectively ‘recessive’ or ‘hidden’. It’s estimated that we all carry some non-working recessive genes, but because we have a second working copy of the gene, we don’t notice any effects.
To give you a visual picture, imagine you're making a chocolate cake. You have two copies of the recipe: one copy has cocoa in it, but the other copy has the cocoa ingredient missing. If the two mixtures are put together in the same bowl, there will still be enough cocoa to give a chocolate flavour. However, if both copies are missing the cocoa, the cake will be different.
If both parents are carriers of one recessive gene variant that causes deafness, but each have a second working copy, they will not have deafness due to this gene. Each time they have a child, each of them could pass on either the working gene copy or the non-working copy.
- There’s a 1 in 4 (25%) chance their child will inherit both non-working genes and have some level of deafness.
- There’s a 1 in 4 (25%) chance their child won’t inherit any copies of the non-working gene, in which case they will not be deaf due to that gene.
- There’s a 1 in 2 (50%) chance their child will inherit one non-working copy and be a carrier like their parents, but not be deaf themselves due to that single altered copy of the gene.
The most common inherited cause of deafness is the Connexin 26 (GJB2) gene, which usually works in a recessive way. Until recently, the only genetic test routinely offered to families with a history of deafness was for Connexin 26.
Read Jessica’s story
Jessica (9) and her parents were given genetic tests after she was identified as being profoundly deaf. The tests revealed her deafness was caused by a mutation of the Connexin 26 gene. Read Jessica's story.
Sometimes, a gene variation can be dominant. This means that one altered copy can cause an effect or condition even if there’s a second working copy of the gene. This is known as autosomal dominant inheritance (‘autosomal’ means that the gene is located on a pair of the non-sex chromosomes).
To give you a visual picture, imagine you’re making potato soup. You have two copies of the recipe, but one has a variation that says tomato instead of potato. Although one of the copies is the original potato recipe, if the two soups are mixed together, the tomato in the altered recipe is enough to give a tomato flavour to the soup.
Dominant conditions can vary in how they affect people. Different people, even within the same family, may have different features of the condition. Some people can carry a dominant gene variation without showing any signs of the condition at all.
If only one parent carries a dominant gene variant that causes deafness, there’s a 1 in 2 (50%) chance that their child will inherit that version of the gene. However, the parent and the child might have different levels of deafness, or one may have no deafness at all.
While most of our genes are carried in the nucleus of the cell, a few are located within the mitochondria. Mitochondria are the powerhouses of the cell: little bodies within the cell that generate the energy that the cells need to function. There are many mitochondria in each cell, each with one copy of their set of genes.
Mitochondria can only be passed on by the mother because the egg is very big compared to the sperm and contains some mitochondria. This means that if a woman carries a gene variant in all or some of her mitochondria, she will pass on this copy to all her children. This is called mitochondrial inheritance. Because the number of mitochondria that inherit the gene variant is variable, the effect of the gene variant will also be variable.
If a dad has a mitochondrial condition, he can’t pass this on to his children.
Conditions with the mitochondrial pattern of inheritance are rare, but certain forms of deafness can be inherited in this way.
Find out more about mitochondrial inheritance from the Genomics Education Programme’s article ‘Meet the mitochondria’.
Most genes that cause deafness only affect the ear, so they do not cause any related medical problems. However, sometimes deafness can be a symptom of a wider health condition known as a 'syndrome'.
A syndrome is a medical term meaning a group of symptoms that are recognised to occur together. Sometimes there’s a clear and known cause of a syndrome: some are genetic and some not genetic.
About one in three children with genetic deafness have a syndromic cause, meaning that they have, or may develop, other symptoms.
There are over 400 syndromes associated with deafness, most of them relatively rare. Most investigations offered to individuals and families with deafness are to identify or rule out, as far as possible, related health problems due to a syndromic cause.
The list below includes some of the more commonly identified syndromes associated with deafness, together with links to helpful resources and family stories.
Barakat syndrome (also known as HDR syndrome)
Branchiootorenal (BOR) syndrome
- MedlinePlus: Branchiootorenal syndrome
- Read about Hannah who has Branchiootorenal syndrome and, separately, autism and mild learning difficulties.
- Contact: CHARGE syndrome
- MedlinePlus: CHARGE syndrome
- Read about Isaac or Joshua who both have CHARGE syndrome.
- Check out our webpage about Down’s syndrome
- Down’s Syndrome Association website
- Read about Caleb or Griffin who both have Down’s syndrome.
- MedlinePlus: Jervell-Lange-Nielsen syndrome
- Read about Primrose who has Jervell-Lange-Neilsen syndrome.
Treacher Collins syndrome
- Contact: Treacher Collins syndrome
- MedlinePlus: Treacher Collins syndrome
- Read about Maia or William who both have Treacher Collins syndrome.